One thing you learn quickly when you have a limited income and a tight budget is take advantage of community events, especially free events.
I'm blessed to live in an area that has weekly free live band concerts, free movies in the park, free weekly kids shows. Library that puts on a good summer reading programs, although my kids are to old for the summer reading program, but do enjoy helping out and running the games, crafts, etc.
The best part is these events are easy on the checkbook, building memories with the kids, and looking back they are going to say hey we did things.
We also take advantage of free community days at the local museums, botanical gardens, and other venues that normally cost money.
I learn about these things via Facebook, local newspapers, and flyers around town. How has your summer been? Do you have community events you can be taking advantage of?
And don't forget the standard stand by. A good day at the play ground. My kids being teens don't care to really play at the play ground, but do enjoy Geochaching and hiking in general.