On the north end of the park, north of the marina, is 'Ding' Darling Shelter House, which is available for rent. It sits on a lake, and there's a playground there that isn't as busy as Union Park.
When we were there (mid-July), there were still plenty of spent fireworks around the parking lot, and the dumpsters were overflowing. So, I'm not sure how much regular care this park gets, but it seems like it would be a great place to spend the day if it was serviced a bit more. The park itself was pretty quiet--much more so than Union Park, which was full of kids--busy with families and day camps!
I think this is a park I would have loved if I discovered it a couple years ago, when the kids were smaller. It's quiet, the equipment is fun but not too challenging, and there's even a little snowplow bouncy thing to ride on.
Below is the Google Maps link for directions. To park at the playground and shelter house, turn at the lot that is just south of Guthrie Avenue.