San Ignacio is the base for inland Belize, and the trip that defines it is Actun Tunichil Muknal — the ATM cave. You wade and swim into the entrance, walk a kilometre of river passage, then climb barefoot in socks through chambers of Maya pots and, at the end, the calcified skeleton the cave is named for. Numbers are capped, only licensed operators may take you, and cameras have been banned inside since one was dropped on a skull. It is genuinely one of the great archaeological visits anywhere, and it is not for the claustrophobic.
The other trips run from town too: Caracol, the largest Maya city in Belize, deep in the Chiquibul on a road that needs the dry season; Xunantunich across the hand-cranked ferry; cave tubing on the Caves Branch; canoeing on the Macal; and the border run to Tikal in Guatemala, which is a very long day but perfectly doable.
Guides are compulsory at ATM and worth having everywhere else — the sites are unsignposted and the roads are worse than the map suggests.