Mammoth Springs State Park is a beautiful state park in Arkansas that's based around freshwater springs that churn out millions of gallons of rock-filtered water every hour. Naturally, boating, canoeing, swimming, and hiking are the order of the day at Mammoth Springs. If you start at the marina, there's a place to rent kayaks and canoes for around $5-7 per hour, cheap enough to spend a few hours paddling around and exploring the park. There are also several trails you can do. The easiest one to find (you can also reach it at the marina) is the Spring Lake Trail which lakes you on a loop around the park. The trail is mostly flat so don't worry about being in super shape to enjoy it.
There are also some rougher trails if you like to do trail-running or mountain-biking. In summertime, it's one of the most beautiful places you can find. The water that comes out of the springs is clean, pure, and deliciously cold so it makes a perfect place to cool off. Enjoy!