When introducing iMovie and all the various bells, whistles, and sound effects that go with it my "go to" activity is the above "How to Blow a Bubble" movie.
The "learning" activity focuses on the use of different transition words with the "tech" activity being the ability to produce a movie using the iPad.
Students LOVE it (Who wouldn't?....You get to chew gum in class!). The entire activity takes about an hour from start to finish and students are required to have seven pictures (includes the title picture), six transition words, their voice recorded, music, and at least one sound effect (in that order!).
They must pair up so that one person can take pictures while the other one goes through the steps. They switch roles so they both make a movie (they make the movie as a solo activity). This takes 10 minutes. I found it incredibly helpful to time them otherwise they take FOREVER to get the pictures done.
Once the pictures are complete they work on the movie. What I found helpful as a tech coach is to travel with a class set of ear buds with built in microphones (I like the expensive iPad ones...$30 each...isn't that insane?!?! My supervisor approved the purchase and I clean them after each use!!!!). This allows 24 kids in a classroom to record without interference AND keep you from hearing them test out ALL the sound effects and music options. Really that was one of the best purchases I have made this year!
If you have to introduce iMovie and aren't sure what project will work...I highly recommend this one.