1. Try golfing, or get a friend to mini golf with you. People who enjoy regular golf can spend a whole afternoon at the course.
2. It's summer, go swimming! Even if you don't live near a beach, most places have lakes to swim at - normally the bigger the lake is, the better.
3. Go bike riding - it's great excercise and nice to be outside.
4. Grab a family member or friend to go hiking with, its always a feeling of accomplishment when you get to the top

5. Find something nearby on ticketmaster - there are usually shows, comedians or something nearby. If you live in a small area, far from a city, like myself, go to the website of you local civic center. A lot of civic centers have shows/events during the summer.
6. Find out whats going on for local festivals/fairs, these are always fun.
7. Get some books - they're great for a day that the weather sucks and nobody is around, and a good book can keep you entertained for hours.
8. I like to make baskets as a hobby. It's relaxing and keeps you focused.
9. Take up fishing, its easy to find a river, lake, or ocean.
Hopefully some of these ideas will keep you offline and out to do some stuff!