Self-Imposed: that is, you have to choose to do it. Going to work so you don’t lose your apartment doesn’t count.
Daily: if you don’t do it regularly, you’ll drop them. You don’t have to do it daily, but daily makes it harder to forget.
Challenging: easy things don’t help you. Watching TV, listening to music, and reading don’t qualify.
Healthy: it has to improve your health or well-being. Smoking doesn’t count.
Activity: you have to physically do something. Thinking about things doesn’t count. Writing does. Meditation does.
I simplify this list of commonalities SIDCHA