Not as classy as some of the other suggestions but the really super cheap way kill a long layover would be to walk out to the street in front of the airport (Harbor Drive) and catch the 923 bus to Ocean Beach, get off on Newport Ave and walk 2 blocks to the beach. No place high class but there plenty of beach area type restaurants along Newport Ave. to get lunch. Same way back to the airport. It's only a 15 minute or so ride each way but note the 923 only runs once an hour on holidays. $2 fare.
http://transit.511sd.com/Services/SD...tePDFs/923.pdf