Along the lines of Write a Backup Adventure, I’ve recently started keeping a fallback RPG ready to go for “gaming emergencies.”
Right now, I have the fabulous new edition of Warhammer Fantasy Roleplay, along with an entire three-book published campaign, just sitting on the shelf waiting to be run for my group. I’m familiar with the rules, and I’ve skimmed the adventures — I could easily start GMing a WFRP campaign on short (or no) notice.
I also know that my group, while they’d prefer a more customized campaign (rather than the “slot in your characters” approach that goes along with published modules), likes WFRP: we’ve played it once before, and everyone had a good time.
Those are the main criteria I recommend when choosing a backup game: familiarity with the system, adventures ready to go, pre-approved (at least to some extent) by your group and easy to run without any notice. There’s one more optional criterion, too, which depends on your group’s tastes: it’s a change of pace, as WFRP isn’t one of the games we’re currently playing.
So if either of our current GMs burns out or needs a break, or if we need a change of pace, WFRP is ready and waiting. I haven’t needed to Break Glass in Case of Gaming Emergency yet, but if I ever do I know that WFRP will be there.