Cycling - A great exercise in the summer.
Cycling to work, to the stores, to visit friends and family can give you all the exercise you need while doing the things that you would be doing in any case.
Start off slowly at the beginning of the summer. Short rides at a slow pace so that you get your cycling muscles in shape before going for long rides.
A bonus is how good your legs will look when you cycle a lot! Nicely toned muscles!