I was almost dreading summer because we have an only child, and now that she's used to having a social life after a year of kindergarten, I was afraid she would drive me batty all summer!

But so far, so good.
We have
swim lessons once a week
made one trip (so far) to the local library
free kids movie day at the theater (check your local Regal Cinema for Free Family Film Festival - free G and PG movies two days a week!)
Farmers Market on Fridays
play dates with school friends
VBS is in two weeks
a few camping trips planned, and a visit to my parents house in the mountains
And she goes back to school on August 18 - so now I'm afraid the summer is going to go too quickly!

Edit: Another fun activity for the whole family, but you need a GPS, is called geocaching. It's like a treasure hunt of sorts and is played world-wide. People hide a cache in some sort of container, post a description and coordinates online, then you go find it! You bring items to trade (kids love this) and sign the log book, then log your visit on the website as well. We always look up a few when we are going to be visiting another city or area - it's a great way to get around a new place and most likely visit places that are off the beaten track, as the caches there will be hidden by locals who know good areas to go to! Check it out!
http://www.geocaching.com/