I have been given money, beers and pastries on several occasions when customers have been happy with me. Once I got a bottle of champagne, after I told the customer I put wrong grease into his coaster hub by mistake when tired, and it started squealing soon when the grease burned off. Customer said “nobody ever admits they made a mistake, so here is a bottle for you!” after I fixed my mistake for free – customer was ready to buy a new wheel or anything, since he knew nothing about bikes.
Nowadays, I do not drink any alcohol, but coffee, tea or vegan candy would work – you know, there are vegan mechanics… But a cash tip may be very well appreciated as well.