I have 2 and can't afford it either, but I'd pay $75 a wk for both my kids to go. Maybe you could find another couple kids to go and that money would pay for your adventures. It would be more fun with more friends too!
I would start by googling free stuff in your area. Buy a big white dry erase calendar and fill in all the free events. We have websites here, My DFW Mommy, Frugal In Fort Worth. I check them regularly for upcoming events. Sometimes I just google - free and put in my zip code. You'd be suprised at how much stuff comes up. See how much it is for season passes or memberships to local stuff. We have a Museum pass here. You buy one and you can go to over 15 museums with it.
You could go to Michaels and get t-shirts, aprons, and visors to decorate. You can buy Iron on transfers, use fabric paint, embellish with whatever, but be mindful that you want something that will last all summer. Or you can start the summer with white shirts and then everytime you visit a place or have an adventure, you can write the name on the shirt with a paint pen or draw a picture that represents it. By the end of the summer they will have a walking collage.
Sounds like a blast. Another idea is to let them join Boy Scouts/Girl Scouts. They have organized activities all summer that are pretty inexpensive.