If you pass through St Petersburg and are heading north (not the most popular destination but worth checking out), make sure to stop by the Weeki Wachee river. It is not such a secret spot anymore but you will still find a way to be pretty much on your own, floating on the transparent spring water.
Just bring a waterproof camera and a picnic for what usually is a 3-5-hour descent. You can either choose kayaks or stand up paddle boards. We chose the latest even though we were complete beginners at it and it was the best! We were able to spot manatees, turtles and otters.
When I say 3-5 hours for the descent, rest assured this requires hardly any effort as the river is gently taking you down the way. It’s 3 hours of pretending you are walking on water while looking at the fish passing by. It’s the most relaxing exercise you can imagine!