Saint Helier is the main city of Jersey.
From the west of the island, you will need to travel either along Victoria Avenue or along the inner road. These are quite busy, and traveling during rush hour can add a considerable amount of time onto your journey.
From the east you will almost definitely need to take the coast road, which runs (unsurprisingly) along the south east coast of the island.
In order to look around the majority of St Helier you will need to park. Most parking spaces require scratch cards which can be bought from any news agents. You will need to scratch off the day of the week, the date, the month and the time you arrive. You will need 1 unit per hour (cards being sold in 1 or 2 unit blocks) and to leave them on your windscreen where they are visible to traffic wardens. There are several multi story car parks and a couple of large open air ones. Crime in Jersey is fairly low, and it is unlikely you will run into trouble leaving your car in any of these places.
All buses in Jersey run to and from St Helier. The major bus routes run approximately every 20mins and the minor ones tend to run about once an hour. The fare is a maximum of £1.60 from anywhere on the island with a bus stop, but the coverage isn't amazing.
It is possible to walk to St Helier from most places, and as long as you know your route you are unlikely to be walking more than 5 or 6 miles from wherever you are.
There is a cycle route that runs from St Aubin to St Helier along the sea front. There are plenty of cafes along the way in case you get hungry as well. You can also walk this route or hire betty bikes in the summer.
The majority of St Helier's centre is pedestrianised and the rest is made up of one way streets. Once you are there it is advisable to walk around to see the place.
There are 3 museums in the town centre, the Maritime Museum, the Occupation Museum and the Jersey Museum. All of these are interesting and worth a look. There is also the waterfront area, which is all reclaimed land and the town itself has plenty of parks and some shops.
This is probably the most exciting if you have children. Located just behind the steam clock, between the avenue and the tunnel, it has several interactive exhibits about Jersey's maritime history, which is a long and quite colourful one, involving pirates!
This is a museum dedicated to the German occupation. This is, obviously, not the most cheery museum, but worth a visit if you are interested in Military History. It is located on the same road as the bus station.
This covers the history of Jersey, and is quite interesting. The first floor has an exhibit on mammoths and a concentration camp exhibit, as well as a cafe, a welcome desk and a shop. There is also a temporary exhibit gallery, and a case displaying some things owned by Lilly Langtree. The next floor up covers Jersey's farming history with some things about the occupation as well as some information about Jersey wildlife. There is also a floor which acts like an art gallery and contains a lot of quite interesting paintings with a connection to the island. Lastly, there is a recreation of a Victorian town house, which if you go in tourist season you may get shown around by a local actress. This includes a child's playroom, with a lot of old fashioned toys as well as costumes for children to play with and dress up in.
There are several areas to the waterfront, and it is a pleasant place to spend an afternoon. The main area contains a cinema and a swimming pool. These are popular with the local residents, however, they are not particularly different from a cinema or pool you could find elsewhere, and if you wish to swim, I would recommend the sea during the summer. There is also a large harbour where you can look at a lot of boats. To get into the harbour you need a code, however, it is easy to look over the fence and view the boats in there. Lastly, there is a large area with a fountain, plants and a (very expensive) cafe. The fountain is great fun if you have small children, and quite a few will be playing in it in the summer.