After reading "the talent code" I have become more fond of constantly changing the challenge on your nervous system. Two of my favorites are:
1. Hold the same stroke count as you descend 10x25. Using the tempo trainer, drop 0.01 or 0.02 seconds per stroke each 25 until you feel you can barely hold on to your stroke count. Then hold that tempo for 25-50-75-100. Only go as far as you can and hold that stroke count and tempo. Notice that here you have a tempo challenge and a maintenance challenge.
2. Sets of 3x whatever (I like 200s). Odd sets descend by holding the same stroke count and swimming faster tempo. Even sets descend by holding the same tempo and dropping stroke count or improving turns.