I did a really fun abs workout in class the other day using noodles and partners.
What you do is have your class partner up with one noodle per pair. Take that noodle and tie it in a knot. The pair will be standing back to back and then they will pass the noodle around each other. Have them do this for a few minutes and then switch (pass the other direction).
Both of my classes had a really good time with this. Make sure you tell them to put the noodle in the water for more resistance.