You won’t find a whole lot to do around Monterrico [1] other than the beaches. Whether from a lack of entrepreneurial initiative or what have you, the recreational options aside from sunbathing, swimming, and walking along the beach are pretty limited.
You can always tour the mangrove-lined canals and perhaps do some bird-watching, but there are curiously no outfitters offering such trips in these parts, unlike elsewhere along the Pacific Coast.
For boat tours, ask at the CECON station for recommended boatmen and tour guides (trained by CECON) who can take you around to see birds and, with some luck, iguanas and anteaters. Some of the beachfront hotels rent boogie boards.
Links:
[1] http://www.moon.com/destinations/guatemala/the-pacific-coast/monterrico