Terri:
Good back up plans are activities (or video/DVD/CD) that require little preparation, can be implemmented on short notice, and don't require too much resources. These could be team challenges, Q&A; jeopardy-style competions; demonstrations; skills practive; group study; student led lectures; work related discussions, etc. Really, hundreds of possibilities.
More effective back-ups will be closely related to the general topic at hand. If possible, we want the activity to appear seamless, as if this is what was planned to start with. Having a few really good back up plans can really save the day when all that was planned just can't happen for whatever reason.
Regards, Barry