Gordon River Greenway is a 3 mile out and back trail located near Naples, Florida that features a river and is good for all skill levels. The trail is primarily used for nature trips and birding and is accessible year-round.
For walking, don't bother with the southern end (the end adjoining the airport). The southern end would be fine for biking, but there's no nearby parking. Instead, if you're looking for a hike, go to the northern end, just off of Golden Gate Parkway. There you'll find a decent walk on a wide paved path. Better yet is the short boardwalk at the northern end.