My friend started attending this one church about a year ago. One of the things the minister advocated is doing something like you described. My friend went through a carwash about 4 weeks ago and she paid for the person behind her. She didn't stick around for a "thank you". She felt good just doing someone a good turn.
I got out a few years after real saxophinist, if you or anyone want to pay my mortgage, be my guest, Please. ;-)
something as simple as buying a cup of joe works swell too.