A great way to enjoy the summer weather is to take advantage of lakes and oceans. The air is usually much cooler near the water and there are so many fun things you can do to work your body besides swimming.
Windsurfing is fun and feels adventurous. You can rent a board, thats what you stand on, and a rig - the sail, a mast and the boom (the bar you hold onto). The board and the rig together are sometimes referred to as a "kit." There is the uphaul, a rope attached to the boom and the bottom of the mast that you use to pull the sail up.
I'd highly recommend getting a lesson from an experience instructor. They can give you pointers on how to position your board in regards to the wind, how to pull the sail up without getting knocked over, how to stand and stay balanced on the board, as well as the simple but important skill of how to steer.
If you want to know more about getting started, then read more
It's a good idea to first start out on land. You've got to learn how to control the sail first without worrying about staying balanced on your board. You can do this by sticking the base of the mast (it has the sail attached) into the sand or ground. Practice holding the mast (sometimes it has a little handle on it) with one hand, and pulling the boom closer to you with your backhand - this is how you catch the wind and start moving.
When you get a handle on that, venture into some shallow water with just the board. Practice standing on it to get a feel for its movements. Then you can stick the mast into the center of your board and let the mast and sail float in the water (it can swivel down). Learning to pull your sail up by pulling on the uphaul rope is a little tricky so don't get frustrated if you fall off a million times. Remember laughing is a workout too.
Once you get the hang of staying up and slowly pulling the boom towards you to catch the wind, you'll be cruising along in no time. It's a great upper and lower body workout because you use all your muscles to try and stay balanced. Since it's challenging, it's a lot of fun, and once you get the hang of it, you'll feel so proud of yourself.
Here's a video explaining how to get your board ready, how to get on it, and how to do some basic windsurfing moves.