Check with your local craft store many offer family saturdays where the kids can go and make a craft together either free or low cost.
I had forgotten all about crafts stores offering projects like this. There are a couple of stores not too far away from us.
Trip to a park for a nature hunt is always fun
Check into some of the cranium games many of them are good for multi ages
Many theatres such as regal cinemas of a free day once a week during the summer to go and take the kids to a movie
I have looked into this one too but it would have to be a planned trip because there isnt a theatre real close to us. This is a good one though that many people do not know of!! 
Home Depot and Leowes offer free kids clinics at least once a month where they can both go and build something together.
Was planning on doing this one---I think that most stores hold the class on the first Sat. of the month. I totally forgot about it. 
Hide and seek is also another fun one
Tie Dyeing a shirt or other article of clothing.
I have been thinking about trying this too. Even the reverse Tie dye. FC has a couple of good articles on this subject! Thanks for all of the tips! BTW, Welcome to FC. I hope that you are enjoying it here! Tami