Recent exercise - swimming laps

I recently took up lap swimming again.  My current workout takes about 35-40 minutes.  I swim 20 laps of freestyle, 10 laps of breaststroke, 10 laps of backstrock, and finish with another 10 laps of freestyle.  I hope to change up my routine periodically but it's working for me for now.  If you are a swimmer, which exercise do you do?