There's a nice playground behind the council of aging in Truro. It's called Puma Park.
Head of the Meadow beach was one of my favorites as a kid. I haven't been in a long time though.
I think Marconi is an ocean side beach, right? I vaguely remember going there as a kid. Honestly, I think all the beaches in that area are nice.
There's an audobon park in Wellfleet too. I haven't been there before though.
Provencetown is fun for a day trip if you have the time with lots of good places to eat.