Flores exists as a base for Tikal, an hour and a half north, and almost everything sold on the island is a variation on getting you there: the standard day, the sunrise trip that puts you inside the gate before the forest wakes up, and the sunset one, which has the better light on Temple IV and a fraction of the crowd.
Beyond Tikal the Petén holds a dozen more sites in various states of excavation — Yaxhá between its lakes, Uaxactún, Nakúm — and, for the genuinely committed, the walk in to El Mirador: five days on foot through the forest, or a helicopter if you have the money.
The agencies cluster on the island and in Santa Elena across the causeway. Prices for the Tikal day barely move between them, because the park fee is fixed and the shuttle is the same minibus.