This one is essential too!
Your mind will tend to get VERY bored if you simply repeat the same approach over and over again!
Yes, it is better to diversify rather than sticking to just one training approach.
For instance, instead of only jogging, you can swim and bike as well.
Instead of only going to the gym, you can train outdoor.
If you trained your upper body, focus on legs, abs and back the following time.
If you went solo one day, train with some friends the following day.
The key is to diversify in any way you want.