There are a number of attractions available to nonguests at The Lodge at Chaa Creek [1] (tel. 501/824-2037, entrance US$10 pp), including this short riverside hike, highlighting the medicinal plants of the Maya and their uses.
The site also boasts the Blue Morpho Butterfly Breeding Center, the Chaa Creek Natural History Museum (with exhibit areas that examine ecosystems, geology, and Maya culture in the Cayo area), and a gift shop.
Touring all of these can take a half day; the office is at The Lodge at Chaa Creek.
Links:
[1] http://www.moon.com/destinations/belize/cayo-district/macal-river-jungle-lodges/left-bank-the-macal-river