Often bars will offer a free beer to the mayor, and I've seen a few restaurants that offer the foursquare mayor a free appetizer when they buy a meal.

I do software consulting and contracting work for a number of clients, and I spend most of my time working in a cafe (free wifi, good chairs, they don't mind me being there 40 hours a week). They have a Foursquare special that gives me a free muffin for being the mayor.