Get movies for free at the library and have a "movie night" on the floor with popcorn, apples, and cheese. Go to the library programs - they have free activites for all ages, even mommies :)
Spend lots of time at the pool swimming and snacking.
Paint with water on the sidewalk, color with chalk, take a bowl of fruit out to the yard and invite all the neighborhood kids to come have a snack, blow bubbles.
Our public parks have "splash pads" that are free, so we did that several times, I just packed a cooler full of fruit and cereal and made a day of it.
When it's just too hot to be outside stay in and make a fort of sheets in the living room, play playdoh, do puzzles, bake cookies, or make homemade popsicles.
We were on a serious budget last summer and we spent most of our time at home, the kids really thrived and enjoyed it. I like being out and about, but my kids just want to be home spending time with each other and me. We made really great connections with the neighborhood kids and moms and just had a blast!