Your best bet would be to find a lawyer to help you draw an agreement up. Our's basiclly reads:
The customer is responsable for the bike.
The customer is responsable for his/her/their own saftey.
Clauses about damage, replacment cost, return policy, and other legal stuff.
A clause regarding local issues.
You may want to include something regarding helmet use, use by children, and storage. Remember, bicycles are regulated as vehicles, you can't enter into a rental agreement with a minor.
Again, find a lawyer. I work for a large hospitality company with lawyers on the payroll. Don't go it alone. I don't think this would cost very much.