Valerie,
Great! A good back-up plan should be as solid as the regular, planned activity. I've found the times I've initiated a back-up plan most commonly is when whatever I had planned is not able to be conducted for whatever reasons. So, for back-up plans, I use activities that are low tech, easy to implemment, related to the subject matter, involves their primary career choice, and is interesting and fun. I don't want students to feel as though I'm just filling time. Also, if done smoothly, the students don't even have to know something different from what was planned is occuring.
Barry Westling