Can you get a tax receipt for giving a charity free rent at your property? Possibly.
You might be able to get a receipt if:
(a) you take the rent and donate it back to the charity (including the rent in income, then claiming the donation tax credit against the tax payable) or
(b) you give the charity a formal lease, giving it a “an exclusive and legally enforceable claim” to use the property. (See esp. paras. 8 and 14.)