Go on a hike!
No matter where you are, I'm sure you have a state park near you. State parks are free to go to and can lead to some good entertainment. Pack up a car, a few water bottles, and get going!
Find a Water Source!
Being from a coastal city, this hasn't been a problem for me. Go find a beach, lake, stream, damn, what have you, and go have a good day. Grab some shades, a blanket, a chair, a swimsuit and have yourself some fun.
Corny, stereotypical, but who cares? Mom is sure to have a basket laying around somewhere. Fill it up with goodies, find a park, and let the feast begin.
Go to your local big city, walk around, take pictures, pretend to be glamorous and the best part...you don't have to spend a cent!
Go on a damn walk!
Get off your lazy butt, go on a walk! Your hometown hasn't changed, but you sure have. Walk around and have a nostalgia trip. You'll reminisce and appreciate what has been in front of you your whole life.
Look at the Stars!
Go lay in a field somewhere, grab a blanket, and talk about philosophy while staring at the night sky. I promise it does wonders for your mental health and is therapeutic.
Now get off your ass and go have some fun!