I'll be riding the Houston-Austin MS150 again this year, so I'll be happy to offer rest stop suggestions!
I'll skip the obvious suggestions re: food and drinks; that all goes without saying and instead I offer the following:
1) Plenty of potty bandwidth -- as many Porta Cans as you can find and have room for
2) A place to sit down -- benches, chairs, whatever. This is an often overlooked aspect of rest stops but is a very welcome feature, especially if your stop is toward the end of that day's ride.
3) Put it on the right side of the road, period. Rest stops on the left side of the road are a real traffic hazard if they are busy stops and riders are getting all crossed up trying to get in and out. Right good, left bad...
4) Shade. Shade is good...
Oh, and by the way... Thanks for volunteering and helping with the rest stop!