I’ve been to lots of grad parties and there never really was a game game.. Most people just like to mingle and do there own thing.

But I have played these at grad parties before..
Tug a war.
Football guys will def. get into that.
Frisbee.
If there’s a basketball court at the park bring a basket ball.
Water balloons.
Egg toss but if there’s no bathrooms near by at this park I would suggest you not do this.

Oh you should have a picture frame or t-shirt or sign or something that people can sign.

Hope your party goes well!