Lake Grapevine is a bit far away for my attendance to be really often, so maybe we could think of an alternate spot that is more central to Dallas.
I've had a Camaro event at night in Highland Park Village before, and the security guy welcomed us because he normally has a boring night with nothing to look at. This was around Christmas 2007 and I advertised it on ls1tech:
A parking lot at Northpark mall could be fun, but I don't know if they would bother us for loitering or not.
Any place on Belt Line in Addison could be fun. Of course, the area is really popular, and parking could be an issue. TGI Fridays holds Gasoline Alley car shows, so they are used to having cars parked there and people loitering.
Other than those places, sort of near me, I can't think of anything at the moment. Parks are always public places, but parking isn't always wonderful. There's a park with a lake/pond connected to Turtle Creek on Lover's Lane near... hmm... can't think of the cross streets, but it's between Hillcrest and Preston. Lots of parking in a single-file row. Great little pond area, I love it there. If you want to throw a football/frisbee or anything else around, it's a pretty darn big park, and good for that.