Thanks for all the great ideas! I love this website. I don't know that I've got an original contribution, but I wanted to contribute to the water aerobics conversation.
Here's a travel combo I use to keep my class focused:
Jack back - Arms (sweep in); Legs (frog kick or breastroke kick) Count the number of strokes necessary to cross the width of the pool. Remember to glide. Long strokes with a glide are the key.
Jog forward - Arms (sweep out or breastroke pull); Legs (bicycle) Double your jack back strokes. (Short, fast strokes)
ex. 10 jack backs = 20 sweeps forward
Counting strokes helps the class to think about stroke efficiency, and in effect, increases the intensity of the workout, and even helps non swimmers feel like swimmers.