Gear the bike for the terrain, I rented a bike on the island of Hawaii once planning to do lots of climbing, the bike was 52/33 front and 11/24 rear, not good for climbing Kohala mountain road, and changing the rear was not an option.
On the plus side, you can give people CO2 inflater with the rental, sell them CO2 cartridges, and if they don't use them, give them their money back in store credits if they turn them back in.
T shirts or jerseys with store or vacation spot logos are nice.
A place to change clothes and store gear, for people just renting the bike for 1 ride.