Hey there again! Remember me??
I would suggest having them first of all design a set for a certain setting. place or time, you know, something like outer space, or the wild west, that they can really imagine a lot of different things. Like a paper and ink sketch. Once their paper and ink is finished, take one sketch and have the class have fun designing a scale model for that set. Watch closely if there’s anything to be changed about how they work (too much paint, hammering to close to edges). You can do this multiple days, with different peoples pictures to liven things up a little!
I don’t know much about rigging, however, so you may want to consult google for that one…
and finally for special effects, this is where they can be the most creative. Bring some odds and ends and then have a mini script written out that you will be reading through. Make sure all the special Fx are written in the script where they belong (but as sounds and what needs to happen). Have them try to puzzle through making all the different sounds, and events using the things that they have (the things you brought with!).
Good Luck and have fun!
P.S. how did your other theater class go?? the the students like the games I suggested?? did you end up using any of them???