Alternating between Upper and Lower Body

If you are swimming and have become somewhat bored with just swimming laps, try switching things up.  Perform whichever stroke you like and only use your upper or lower body.  For example, begin by warming up with 2 regular laps.  Then just use your legs one lap, then just your upper body for another lap, and back to 2 laps with your whole body.  This will put more pressure upon your legs and arms, making you work your cardiorespiratory system more and stress your muscles differently as well.  As you get better, you can perform more laps with just your arms or legs.