If the job is what I think it is, it'll take him longer to walk to the piano than to actually do what he has to do. The bulk of the time will probably be talking to you.
If he's really in town anyway and you don't occupy a large block of his time, slipping him a $20 bill seems appropriate. If it really does occupy 20 to 30 minutes, I'd consider at least doubling that. Perhaps be flexible and observant.
He sounds a little like me, willing to just do something small like that more as a gesture of good will, especially to someone who is already a customer that you'll likely see again and again. Plus, sometimes on little stuff like that, it takes longer to write up a bill than to actually do the job at which point you'd rather not be bothered with the paperwork.
Just my thoughts. $20 minimum. Beyond that, your call.