Have you been here yet? It's a freshwater lake with a beach about 50 miles north of SLC. There are free cabanas, picnic areas, and camping areas. I think it cost about $10 to get in.
It gets CROWDED on the weekends, but everyone generally seems to be in a fantastic mood (possibly because a lot of people are drunk?) (I'm guessing?), so at least they are happy crowds.
Although maybe it's just that the heathens come out on Sunday, which is when we went. (TOTALLY KIDDING HEATHENS OF UTAH.) (DON'T EMAIL ME.)
I love it and can't wait to go back.
Info here: http://www.utah.com/stateparks/willard_bay.htm