Do they allow one to play any fun games like flag football, volleyball or mini soccer Also do they go with walk-ins or needs to reserve?

I would be surprised if they allowed that sort of activity. It's a more sedate crowd. There's also fairly little level ground for sports, as the vast majority of the open ground is on a noticeable incline.

Thanks for the information I guess since I have a large group of people then the best is to reserve

Walk ins are welcome and the field fills up pretty fast with people picnicking. It's first come first serve but they take reservations for large groups. That might help!