I heard about a great park recently. It's called Nelson Park and it's in Plymouth, MA.
It sits right on the ocean.
It has a great splash area for the kids. They love running through the spray.
There's also a playground.
Here's Kelly and her youngest, Sean.
We were fortunate enough to get a nice shady spot.
There's no physical address for the park (that I could find). For GPS purposes use 225 Water Street, Plymouth. This is an apartment (or condo) complex. The park is just after that but before the bend in the road. They have bathrooms and a snack bar where they sell hot dogs, soda and ice cream. We find that it's best to bring a picnic. We pack a cooler and spend a few hours there.
Your best bet is to get there early, the parking lot fills up fast. Oh, and it's totally free!