O.L.
It's not a club, but you can find lots of fun things to do for the summer at www.kidspages.org. They've got a calendar loaded with fun stuff to do.
My own kids are much younger, but they've really enjoyed some of the free concerts & things around town. Also check out www.scfd.org...several of the museums have days that are free to CO residents so you can check them out.
Most libraries have some activities over the summer, and we also do a fair amount of activities through the local recreation centers. Very fun & my kids get to meet lots of other kids that way!
Best of luck & enjoy Colorado!