I haven't visited Gleason Park yet, but on Google maps it seems like it has some strong possibilities. I learned about it on the Reddit /r321 pages.
It is a 27-acre green space with a recreation center, outdoor pool & 2 playgrounds, plus a lakeside trail.
There are various things that you can rent, but I'd recommend visiting first and then talking to the staff. I just talked to a person on the phone that indicated that a wedding location would be $55. I'm not really sure what that entails...