You might want to use a storage facility contract/waiver as a template. They explore all sorts of eventualities related to damage, theft, what happens if the renter stops paying, etc. You might also insist that the renter has the items insured himself which lessens the likelihood he comes after you if something happens.
As Greg indicated, it's probably easiest (and free) to chat with your insurance agent about it. If you expect this to be an ongoing business, it might be worthwhile to talk to a low-cost lawyer just to make sure your bases are covered.
