G.T.
Have you considered renting one of the rooms at Wegmans? I don't know what the cost is, but the one here in Canandaigua is pretty helpful. The Wegmans here in Cdga also has a patio for dining and BEAUTIFUL walking trails with water and bridges behind the store (I walk there with my son all the time). Also, not sure when the party is, but the facilities down near Kershaw Park in Canandaigua are pretty nice. There are some covered picnic areas. Another spot is Onanda Park (you guessed it: in Canandaigua), but there are lodges and covered picnic areas there as well as playground and places to swim and hike.
Hope this helps!
G.