Peter, I've personally stopped in Luxemburg for fuel when we flew home from Aero show. My friend goes there in his JetProp whenever suitable.

When I go up for an hour or so with no real mission to go somewhere specific, I always think about where to refuel if necessary. We have this voucher booklet from airshampoo that someone mentioned which I use as well.

So, there are quite some pilots out there that do exactly that: check fuel prices and landing fees in order to make it their mission of the day. Leer-Papenburg (EDWF) is such a destination. Very friendly people, fuel price is ok (there are better prices in germany) landing fee is waived when you buy fuel.

Yepp, cool point: Whenever I miss a reason to fly, flying about one hour to get cheap fuel is a good reason to fly. It excuses the madness of running an airplane.