Possibilities:

1. One day club membership
2. Skip the club, eat at the airport restaurant, the ones in the United terminal area
3. Leave the airport, negotiate with a taxi to drive you around for a fixed price.
4. Rent a car, possibly cheaper. Not as good if you are driving into San Francisco.
5. Take a bus or BART into downtown San Francisco.
6. Visit things outside of San Francisco, such as UC Berkeley, Golden Gate Bridge, USS Hornet aircraft carrier, Stanford, Hiller Aviation Museum, Muir Woods, Presidio, etc.