Car rental is the most practical way to explore Guam. The island is only about 30 miles long and 9 miles wide, so you can drive from one end to the other in under an hour. All the major international companies — Hertz, Budget, Avis, Enterprise — have desks at the airport and in hotels along Tumon Bay. A valid US or international driver's license works fine. There's no Uber or Lyft, but a local app-based service called Good to Go Rides operates similarly and can be booked online.
Public buses are run by the Guam Regional Transit Authority (GRTA) and serve most of the island's villages. The routes weren't designed with tourists in mind, so schedules are infrequent and connections can be awkward. Buses are fine if you're patient and your destination happens to be on a main route.
Taxis are metered and available at hotels and shopping centers. The flag rate is $2.40, then $4.00 for the first mile and $0.80 per quarter mile after that. Drivers occasionally try to negotiate a flat rate that works out higher — insist on the meter.
Cycling is possible around Tumon Bay, where some hotels and shops rent bikes, but the rest of the island isn't bike-friendly. Traffic on Marine Corps Drive (Route 1) moves fast and there are few dedicated lanes.