Charleston sits at the mouth of South Slough, a tidal river basin that turns into a vast, muddy estuary full of wildlife. Four miles south of Charleston on Seven Devils Rd is this excellent interpretive center, with exhibits on estuarine ecology, also showcased in a 12-minute video. It runs events and programs all year.
Several walking trails, ranging from 0.25 to 3 miles, offer glimpses of the local ecology. Canoeing is also possible but you'll need your own canoe.