From the Upper Peninsula, you have to cross the Mackinac Bridge, via St. Ignace. The "Mighty Mac" (a certain burger vendor objects to another nickname) is 5 miles long and at its highest, 200 feet from the surface of the Straits of Mackinac. The toll is $1.25/axle for passenger vehicles (that's $2.50 for a car or motorcycle), $2/axle for motor homes, and $3/axle for anything else. The drive can be a little intimidating, but only two vehicles have gone over the edge in its half century of use: one a lightweight Yugo caught by very high winds, the other an SUV in a suicide; both were speeding. If you're uncomfortable driving across, a bridge authority employee will happily drive your vehicle for you (including motorcycles--they love Harleys). Just ask the toll collector. The bridge closes to traffic when the winds exceed 65mph, but is otherwise open all the time.
The car ferry service that connected the two peninsulas before the Bridge was built is no longer available.
Although pedestrians, bicycles, and snowmobiles are not permitted on the Mackinac Bridge, the bridge authority will take you across, for $2 per person and per bicycle, or (8AM-8PM only) $10 for each snowmobile and rider.
This could be interesting, if you somehow got to Mackinaw City without your own vehicle your options are really limited as there is no public transportation. Of course, the town is quite small, and pretty much all tourist attractions are within walking distance.
The Mackinac Bridge isn't just a way to get to and from the Upper Peninsula; it's a sight to behold. Envisioned since the late 19th century, it was finally begun in the 1950s and completed in 1957. The two towers are visible from almost any high ground in the area, and watching the illumination of the bridge's lights is a nightly ritual akin to watching the sun set. Despite its size, the bridge was never record-breaking since suspension bridges are usually measured from tower to tower (the suspended center span) and the record belonged at the time to the Golden Gate Bridge in San Francisco. For that reason, the bridge was formally dedicated as "the world's longest suspension bridge between anchorages", record it lost to other bridges in Asia, hence the revised title of "the world's longest suspension bridge between anchorages in the Western Hemisphere". The bridge has now the 10th longest center span in the world and the 3rd in the United States.
This is the ideal jump off point for trips throughout not only the tip of the Lower Penninsula but a large area of the Upper Peninsula is within an hour or two drive time from Mackinaw City. There are shops spread all around town that for the most part offer the standard tourist fare, nothing too extravagant.
We can't let the fudge shop go without being mentioned as this seems to be the Straits area's largest export. Partake of the free samples and when you find the one that you like buy it. To say that one is better than the other is going to lead to countless and eventually futile edits so lets leave it at that.
Both in town and and just outside town there are historical features waiting to be explored, from Fort Michilimackinac to Old Mackinac Point Lighthouse on to Old Mill Creek will take you the better part of the day.
Day two would be the day for the trip to Mackinac Island via one of the many transit companies available to you. More history waits for you there all over Mackinac Island. Also once again there is fudge to be had, the fudge shops there realized some years ago that by installing exhaust fans over the front door to the shop they could entice more prospective customers into the shop. Be warned that since the primary mode of transport for goods and people on Mackinac Island is the horse there is usually also a far amount of "spent horse fuel" about so this can indeed make an interesting combination of odors. The island does maintain a fleet of hardy souls that follow behind the horses to try and keep the streets clean. Given the number of horses and the number of scoopers they do an admirable job. You can walk, bicycle or take a carriage tour of the island to get around. Those that are used to horseback riding can visit a livery and rent a carriage if you wish. History abounds as they say, enjoy the day and watch were you step.
On day three it's time to go to the Upper Peninsula to so some sight seeing. The Soo Locks are about one hour past the bridge on I-75. The city is actually called Sault Sainte Marie, and to confuse things just a bit there is one on each side of the border there in the U.S. and Canada. The Soo Locks enable the shipping that transits the Great Lakes access to Lake Superior when travleing in the upbound direction and the materials from the shipping ports of Lake Superior access to the rest of the world without being off loaded at the St. Marys River where the elevation between Lake Superior and Lake Huron change. Stop at the locks and watch the ships go by, to those of you from elsewhere in the world they are ships, you will find that most native born Michigan residents call them "boats" and the ocean going vessels plying the lakes are "salties". Just as a fun fact a "saltie" is required to take on a lake pilot when entering the Great Lakes. Head west from the Soo over to Whitefish Bay and take in the Great Lakes Shipwreck Museum there and amble to Taquamenon Falls to finish the day. A stop in Newberry on the way back to Mackinaw City for dinner and then back to your base in Mackinaw City.
Lets face it by day 4 you need a vacation. You may have a hotel with a pool or one on the beach, kick back and take it easy. If no beach or pool Mackinaw City does have a Municipal Beach, take Central Avenue west out of town make one left turn and if you don't stop you will end up in the water. Continue along the lake and it will take you to Wilderness State Park one of the most popular in the state. There are beaches there along with hiking trails if you still want to do some discovery. A daily or annual permit is required to enter the park, a few dollars for a day or at this time $24 annually.
Running Deep Fishing Charters  has fishing trips on Lake Huron. Otherwise you can fish off of the breakwall downtown, which is accessible from the Straits State Harbor.
There is a water park that was just recently built a year or two ago in Mackinaw City if you prefer more of the creature comforts.
Historic Mill Creek Discorvery Park. While still retaining its demonstrations of the historical saw mill and the miles of great hiking, Historic Mill Creek has added a popular new attraction to their park. The Adventure Tour is a guided tour that shows off the natural wonders of the park in and fun and exciting way. Visitors are taken up across an open bridge, getting a tree's eye view of the creek. Visitors also have the opportunity to climb up a climbing wall and explore the different birds that inhabit the trees of Mill Creek. The tour culminates in it's most popular attraction, an exciting ride down the zipline. Flying like and eagle, visitors soar across the Mill Pond and land safely on the platform across the pond. edit
Dining of all kinds are available from peanut butter and jelly to surf and turf. Prices range from moderate to expensive. You will find everything from a small diner just west of I-75 on Central Avenue to fine dining with a fairly decent wine list.
The Mackinaw Pastie Company
Big Stone Bay Fishery just south of town on Stimson Road. Fresh lake perch, this place ships fresh fish to dining establishments throughout the U.S.
There are probably well over one hundred hotels, motels and combinations of motels with cabins in the immediate Mackinaw City area. As rule closer to the Mackinaw Bridge or Lakes Michigan or Huron is more costly. The farther from town the less costly and fewer amenities. The farther away from peak season the less costly.
Mackinaw City Visitors Bureau (Mackinaw City), 10800 Us Hwy 23, ☎ (231) 436-5664, . Mackinaw City Visitors Bureauedit
For those of you that enjoy camping there are quite a few private campgrounds in the area as well as one of Michigan's most popular state parks.
At well over 8,000 acres with over 200 campsites Wilderness State Park has been one of the most popular parks in the state for many years. They offer camping for everything fromn pup tents to motor home in one of three campgrounds East or West Lakeshore or the Pines campgrounds. There are hiking trails that take you through a number of areas in the park.
There is another state park just over the bridge, Straits State Park and a couple of more South of the Straits in the Cheboygan area.
Mackinaw Mill Creek Camping  has views of Mackinac Bridge and Mackinac Island on one mile of shoreline on the Straits of Mackinac with 600 camp sites for everything from large RV's down to small tents. Beautiful new summer lakefront cabin and cottage rentals situated over 200 acres.