I work at the U of R, so I know Genesee Valley pretty well. There are a decent number of parking lots, so long as we were there during the day it should be fine (don't go there at night). I haven't been in Highland in a while, but check the date of the Lilac festival, I imagine it might be crowded. I have also used Mendon Ponds and Powdermill Park for past meets I have organized for another car forum. Powdermill is probably a slightly better location but was crowded when we were there, Mendon Ponds varies.
None of these are state parks, so no entrance fee. If you want to reserve a pavilion it would cost money, but we usually just go find an empty one when we need one.