As with the rest of Belize, English is the official language, but as Cayo is the main entry-exit point for Guatemala you should expect a higher proportion of Spanish speakers here.
The Western Highway cuts through the middle of Cayo, running from the Guatemalan border in the west all the way to Belize City in the east, via Belmopan. The Hummingbird Highway links Belmopan with Dangriga in Stann Creek.
From Cayes: $8 BZE Taxi fare to/from water taxi and bus terminal. Buses to Cayo read BENQUE also possible to catch an express bus to Belmopan (although the sign might read Punta Gorda or Dangriga). Once in Belmopan, either take a taxi, have a beer, shop in the local open market, or wait for the next bus to Benque unless you are already on it! Food vendors will likely be selling drinks and food once the bus stops. Try to use the bathroom first because the bus will leave without you...
From Airport: Taxi fare is usually around $100-$150 USD!!! depending on where you need to go. Shared shuttles are available at a reduced cost compared to a private taxi via William Hoffman's shuttle service. Private taxi's allow for a stop at the Belize Zoo for an additional cost, highly recommended. Only way to go by bus is walk (or taxi) to the Northern Highway (1 mile along airport entrance road) and catch a bus to Belize City. From there transfer to a bus headed to Benque. This will take a bit of time, so plan on this only if your flight is arriving before noon.
From Guatemala: Taxi or walk from border to Benque bus terminal (1 mile) and board buses headed to Belize City (sign will likely say just Belize. San Ignacio is the first terminal stop, Belmopan is about 45 minutes further.
Also it's possible to fly $$$ to Belmopan via Tropic Air a great way to save time if you can afford it.
Many taxi's available in San Ignacio and Belmopan, much less outside of the major towns. The Western Highway has regular bus service (30 minutes to 1 hour) in each direction for a very reasonable cost. Car rental is mostly unnecessary and expensive unless traveling with a large enough group. Gasoline currently at $6 USD a gallon.
Outside of the Western highway, most of the roads are unpaved and rough. Mountain Pine Ridge road especially. Consider this when planning tours to Tikal and ATM Cave, as the travel time increases dramatically over the bumpy dirt roads.
Day 1: Stop at the Belize Zoo coming from airport by taxi or Cayes with backpack on the bus.
Night 1: Stay in San Ignacio if you are single and/or looking for more nightlife (very loud and late),tourist shopping, and "city" living, or one of the many jungle lodges located in the surrounding area (better for adventure seeking couples).
Day 2: ATM Cave tour (around $85 USD) Number 1 highlight in the area, and way more of an adventure than most anticipate. Must be agile, willing to walk/swim (vests available) through water to witness many Maya sacrificial cave offerings. Must see if you are in the area. Consider proximity to the cave when choosing your hotel if you are only staying in western Belize for a day or two. Nice nearby hotels include Pook's Hill Resort , Lower Dover Field Station , and Amber Sunset Jungle Lodge .
Day 3: Tikal Maya site (Guatemala) No explanation necessary! About $140 USD for guided tour, Belize exit fees, lunch, transportation, and pickup/drop off from hotel...Can also be done by yourself for budget travelers by bus but requires more travel savvy and knowing a little bit of Spanish (Save 1/3 to a half). If you go yourself, stay in Flores for the night, that way it is possible to see both the sun rise and sunset at Tikal, also this will allow for less tourist traffic in the early am and late afternoon.
Day 4: Calico Jack's Zip line and cave adventure if you have your own rented vehicle, otherwise it is $60 USD+ to pickup/drop off from lodges. Without vehicle, see Xunantunich , Cahal Pech , or Lower Dover Maya sites via local bus service. Each site requires between a 1/2 to 1 mile walk from the highway to reach the front gate. Xunantunich offers stunning overlooks of Guatemala and the Belize village of Benque along with many howler monkeys sounds and photo opportunities. Cahal Pech overlooks San Ignaico but most of this view is blocked by large trees. The architecture here consists of arched walkways, a unique feature in the area. Lower Dover is a recently identified (unexcavated) Maya site located within the jungle of Lower Dover Field Station's hotel property. The newly identified site is located immediately in-between Barton Ramie and Floral Park along the Belize River. Noted Harvard archaeology professor Gordon Willey and his followers failed to identify this major site during his pioneer settlement survey of the area in the 1960's through the present.
Day 5: Bird watching in AM at local jungle lodges, Guanacaste National Park, or Blue Hole National Park. Canoe down the Belize River or along the Mopan or Macal Rivers to "branch mouth" about $60US pp
Day 6: Mountain Pine Ridge including Caracol. Keep in mind the road is very long and rough to reach this impressive site. The main temple of Canna is still the tallest building in Belize today! Don't go hungover, you will lose it on the road.
Day 7: Saturday Fresh Market in San Ignacio and travel to Cayes via express bus to Belize City.
Cayo is mostly about the jungle experience, but there are also historical and cultural attractions. Visit Mayan ruins at Cahal Pech, Xunantunich, or Caracol. Go caving. Experience the rainforest, watching birds, mammals and insects, at one of the many lodges. Tube down the rivers. See the canopy on a zip line tour. Go horseback riding on a jungle trail. Go to the Saturday market in San Ignacio.