August seems to be best for the weather. The Summers heat has dropped a bit, the fall storms haven't moved in yet and we get temps in the 70's at night.
Another thing to note is that school starts here in WA at the end of August, that excludes many from coming after Aug 25 or so.
I would not be opposed to the Naches Trail, funny rocks area. There is ton or room, it is central located and there are developed areas (water restrooms etc.)with little to no fee required.
The trail system is larger and varies from very technical trails all the away down to beginner with a stock vehicle. You can go from running mud, to difficult rocks, then right on over to a easy trail ride.
I will volunteer myself for whatever is needed most, from a camp host all the way to trail guide. if the event is held close I can count on at least two more mature adults as well.
They hold the NAXJA meet up there yearly and there's plenty of room and fun for all.