San Ignacio, Belize - in fact most of the Yucatan, is made of rock that lends itself nicely to the forming of caves. Mayans believed caves were entrances to the underworld and sacred places. When you combine these two things you get some of the neatest caves in the world.
Actun Tunichil Muknal (ATM) is the single most popular tour in San Ignacio. It's a full day tour and starts with a jungle hike across a river several times, followed by several hours, with headlamps, wading around in the land of Xibalba.
Barton Creek Cave is a half-day trip (but can be full day), and it's one that most people are able to do. Unlike ATM where you can walk around, Barton Creek Cave is completely done in a canoe because otherwise you would have to swim. There are some great formations and Maya artifacts to be seen.
Cave Tubing is a full day trip. The tubing is done at a place called Nohoch Che'en Caves Branch Nature Reserve, where you'll find a series of stunning water filled caves. Because of the low ceiling you are provided with helmets and life jackets. This trip lends itself to a variety of ages.
If you are interested in doing some caving, private trips can be arranged to lots of really cool places. These are all arranged on an individual basis.