If you want to mix up the swimming could do sprints on a set time cycle. throw in the different strokes.

One example of a good workout might be 20 sprints - medley order (butterfly, backstroke, breastroke, freestyle) X 4 - we'd do 25 m sprints with 75 m slow back to the start (but you could make that shorter in a smaller pool) and then start next sprint at the end of the time you've chosen to cycle to eg 2 mins.

Done properly you'll feel it :-)