If you have a regular host site, stock the place with several board and card games. Red Dragon Inn, Munchkin, Conspiracy Theory, and Castle Panic to name a few. Nothing wrong with some old style card games like Bridge, Spades, Hearts, some version of Poker, etc. One way to start the collection is have every player contribute one or more games. Many of us have one or more games in our stash that haven't seen play time. Good candidate to contribute to the Plan B collection at the prime game site.
Nothing wrong with having a Plan B RPG with characters already generated. Since you won't know who will be gone, the GM will have to be flexible on party composition. Good chance for someone other then the normal GM to get some game running time.