Hmmmmmmm 7-12 that a little out of my experience range.
You might check to see if the movie theater near you offers 10 am kids movies for super cheap or even free in the summer.
Look for Vacation Bible Schools in your area.
Some stores like Micheals Arts and Crafts, Lowes and Home Depot offer short little classes for kids. LIke at Micheals, they supplied the beads, key ring and fishing wire and my kids could make a personalized Key Chain. Lowes I think for $8 or less will give you the supplies and let you use tools to make a bird house.
Girls that age would probably love to learn to knit or crochet, There are tons of books you can get from the library. Our library even has a free summer reading program for kids that age where they earn prizes for just checking out the books. Gosh i bet there are tons of fun craft type activities, I liked Origami at this age, craft foam is always fun, scrapbooking,lanyards, etc Google kids crafts and search the web for things that your kids might like.
That reminds me,I bet books stores, like barnes and noble, borders, half-priced books might have reading programs too where they can earn prizes or more books. Just call and ask.
Check out all your local parks and vote on which ones are the funnest, the shadiest on a hot day, the cleanest etc.
HAve a summer fashion show, and go through their closets and get rid of clothes that don't fit.
Have them plan a meal and cook it with supervision. Mix up a new dessert.
Have a yard sale, have a lemonade stand. Make a fort in the back yard, Let them play and come up with some fun things to do.
last thing, check out www.flylady.net for ideas for CampGonnaWannaFly it's ideas for things for families to do during the summer that helps to build routines for maintaining a clean and funcioning home.
Have a Super Fun Summer, and God Bless you for showing your love to these girls.