Watkins Regional Park
I've always been a huge fan of parks. There is a ton of open space and they are usually free. We recently discovered Watkins Regional Park in Upper Marlboro Maryland when my daughter's class had their end of the school year field trip. It was a short drive from our home and it was huge. They have tons of walking trails, pavilions, mini golf, a train, and our favorite, a Wizard of Oz themed playground. There is even a yellow brick road! It also has an indoor nature house when the kiddies need to cool off and play.
It's a fun place to spend with the family with a variety of things to do that range from free to a small fee. There are also a few grills to have a cookout. This place screams Summer for us.
If you're in the area, I highly recommend taking the kiddies to have a good run around. This place is a lot of fun.