We have a trampoline that is getting lots of use now that the weather is warming up. If you have a place for one, it's a great investment. Also, my dc are learning to jump/skip rope. Similar to JennyMN's dc, they were inspired to skip rope because we are reading The Secret Garden. Riding bikes up and down the street is another favorite activity. Thankfully, we live on a street of grandparents, so I never question my kids' safety. Plus I can see all the way to the end of the street from my kitchen window.
For a project, we planted seeds and are anxiously watching them grow. It is the kids' job to water and take care of them. In between our neighbors' homes, our kids have found a little nook to play in. It's totally secluded and shaded with ivy, rocks, and leaves. They have claimed it as their own "clubhouse". It's pretty cute! They have even used the rocks to set up office spaces for themselves, complete with rock computers and mouse pads!
Sidewalk chalk, playing in the water sprinkler, having a lemonade stand, going to the swimming pool, splash pad at our park, pogo stick, and just playing with rocks and sticks and bugs are normal summer activities here. Plus, we live where it gets very hot, so most summer afternoons are spent either swimming or indoors reading, playing board games, or doing craft projects.