We usually participate in our library reading program as well. Time and money permitting I plan mini-vacations (1-3 nights) throughout the summer to nearby cities with a science center, natural history museum or zoo. We have memberships to our local ones so admission is usually free. This way my kids aren't looking for one big vacation, but lots of little ones. We usually research the city, the museums, history and points of interest on the way.
We live close to a national park and they have many resources through there we participate in. They also have letterboxing and geocaching sites. We have done letterboxing, but DH has looked into geocaching as a fun family activity.
Depending on your geographical location you may be close to fossil sites too.
Then of course there are the regular summer favorites like swimming, riding bikes, camping etc.
Hope this helps!
Jen
