I haven`t done this myself personally, but I`m sure a 50$ gift card for groceries or something would be appropriate. Anything less seems unsubstantial, even though it doesn`t sound like there was a major impact to the tenants.
I`m no lawyer, but I cannot see how you would be legally obligated to provide them anything unless you knew about the furnace issues for a long time and did nothing about it. If you have been scheduling regular yearly maintenance on the furnace, and only just found out about the issue (which you resolved within 2 days), it sounds like you`ve fulfilled your landlord accountabilities.
If you`re really all that concerned I would give your provincial landlord/tenant board a call to verify.