A good group activity, time filler, or active learning tool is to have the students make the class kind of like a gameshow. They can throw it together in five minutes, or you can plan ahead and have them plan it for a later class period. They can write questions, or answers, and play it like Jeopardy. Candy makes great prizes! Another thing is to have them work together to make up fun mnemonics to help them remember things. Prizes for the best or funniest ones can be distributed. It can be pretty entertaining.