We had a couple of hours to spare on our drive from marco island to tampa so we looked for things to do on tripadvisor. The six mile cypress slough preserve came up as a top suggestion so we stopped by. It turned out to be the highlight of our trip. As others have said, you can't beat the price - only $1 for parking but I would have gladly paid $20 for admission.
We walked though the preserve on a raised boardwalk which was very easy to navigate. On our walk, which took about an hour including short stops along the way, we saw a few alligators, egrets, woodpeckers, herons, baby wild boars, turtles and much more.
Definitely worth stopping by.