Rice: At thebeginning of the film it's the wedding of Ralph Hapschatt and Betty Munroe. As the newlyweds exit the church, throw the rice along with the on-screen wedding guests.
Ring: When Brad asks Janet to marry him, you can do the same thing with your sweetie.
Newspapers: When Brad and Janet are caught in the storm, Janet covers her head with a newspaper, The "Plain Dealer." You don't want to get wet, do you?
Flashlights: During the "There's a light" verse of "Over at the Frankenstein Place, "you can light up the theater with flashlights or your cellular phone and similar devices. Sorry, no open flames--remember you are wearing newspapers on your head!
Rubber gloves: During and after the creation speech, Frank snaps his rubber gloves three times. Later, Magenta pulls these gloves off his hands. If snap your gloves in sync each time, you'll create a fantastic sound effect.
Noisemakers: At the end of the creation speech, the Transylvanians respond with applause and noisemakers. Hip, hip, hooray!!!
Toilet paper: When Dr. Scott enters the lab, Brad cries out "Great Scott!" At this point, you can hurl rolls of toilet paper into the air (preferably Scotts, because it's common sense on a roll).
Toast: When Frank proposes a toast at dinner, members of the audience throw toast into the air (dry, not buttered...otherwise things could get a bit sticky).
Party hat: At the dinner table, when Frank puts on a party hat, adorn your own head, and join the party.
Bell: During the song "Planet Schmanet Janet," ring the bell when Frank sings, "Did you hear a bell ring?"
Cards: During the song "I'm Going Home," Frank sings, "Cards for sorrow, cards for pain." At this point you can shower the theater with playing cards.