Hey - I have to give up about 16 hours of every weekend for sleeping.  I'd like to know how much I can charge for that; and whom do I bill? 
 
Seriously, though - I'd charge him for the gas and wear/tear on the car, which is $.36/mile at my present job (probably not enough these days).
Anything over that is unnecessary, IMO.  Time - didn't you say you'd take your wife and make a day of it?  You want paid to spend time with your wife?  And food - don't you normally eat on Saturdays, anyway?  Granted, you can eat at home cheaper than dining out, but hey - consider it part of "making a day of it."  Or pack a sandwich.
ps - by your secrecy, I'm assuming this is something along the lines of an original Star Wars movie poster?  
