Fun Practice Idea

Do your swimmers need a break from the repetitiveness? Is is someones birthday? Maybe it's post season and you're trying to keep it interesting for them. Try this idea, we always enjoyed it when we were swimming.

You as the coach assign the warm up but after that the swimmers rule the pool! (sorry had to say it that way)
You can start with the birthday swimmer or the swimmer the next birthday, do it alphabetically, whatever floats your boat. Each swimmer get's to pick a set that the entire team will do. They can make them as fun, crazy, or hard as they like, just don't let them make it too easy like doing 25's for the entire practice. It lets the swimmers have control of practice, of course with you there to supervise, guide decisions and veto too simple sets :)
Let them have fun with it, I'm sure they've been working hard and will enjoy being the ones in charge for the day.