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?