Belize is small enough that most destinations are reachable in a day or less from Belize City.
Buses are cheap and run between all major towns along two main highways — the Northern Highway (toward Corozal and Mexico) and the Western Highway (toward San Ignacio and Guatemala). The bus hub is Novelo's terminal on West Collet Canal in Belize City. Express buses are faster; regular buses stop anywhere you flag them down. The longest route — Belize City to Punta Gorda — takes five to seven hours and costs around BZ$25. Buy no ticket in advance; pay the conductor onboard. Get to the terminal early; schedules can be loose.
Shuttle vans are faster and more comfortable than buses, and popular for the Belize City–San Ignacio corridor and airport transfers. They cost more but save time.
Water taxis are the main way to reach the cayes. Two competing services — Ocean Ferry and San Pedro Belize Express — run multiple daily trips between Belize City and Caye Caulker and Ambergris Caye. The dock is at the Marine Terminal on North Front Street. Trips to Caye Caulker take about 45 minutes; San Pedro takes about 75.
Domestic flights on Tropic Air and Maya Island Air connect Belize City (both the international airport and the smaller Municipal Airport on the waterfront) to San Pedro, Caye Caulker, Dangriga, Placencia, and Punta Gorda. They're pricey but fast — a flight that would take most of a day by bus and boat takes 20 minutes.
Car rental makes sense if you want to explore the interior at your own pace, particularly around San Ignacio and the Mountain Pine Ridge. Roads in the main towns are paved; elsewhere they can be rough. Drive on the right.