DCwom , do you think the western side of the Erie Canalway might work for the OP's purposes? I have never ridden it but on the map it looks like it goes through a lot more small towns than the GAP, so there would be more opportunities to use a regular toilet and get ice cream or do other diversions. There also seem to be a decent number of campgrounds and it goes right by Rochester Airport so that might be a good stopping point and a place to pick up a rental car to get bikes and gear back to the starting point.

In lieu of planning my own bike ride, I got really into this and came up with a theoretical itinerary that seemed like it might work for the OP's plan:

Day 1: Drive to West Canal County Park in Tonawanda, leave car and ride to Niagara County Camping Resort - 20 miles
Day 2: Niagara County Camping Resort to Dollinger's Motel in Albion, NY - 27 miles
Day 3: Albion to Brockport Welcome Center (offers camping and restroom/shower/laundry) - 15 miles
Day 4: Brockport to Rochester International Airport - 20 miles, and actually looks pretty easy and safe to get from the bike path to the car rental area.