I try these challenges often! As a corporate worker, I can usually fit self-directed routines into my life without much trouble. "Don't break the chain" psychology also works well on me.
When I was a scrawny 25 year old, I started doing a 7-minutes-per-day fitness challenge with help from an app.[0] This was easy enough to do before jumping into the shower in the morning, and the privacy was nice since the amount of push-ups I could do without visibly struggling was truly shameful. After a few months I felt I was able to do these basic bodyweight exercises very easily, and I was ready to join a strength training group at the gym. I finished the 7 months anyway, even though I outpaced it when I started using weights. Nowadays I just hit the gym two or three times a week, and have been doing this for a couple years.
I also use Duolingo[1] and have streaked into the hundreds of days a few times. It's fun and easy to do, but unfortunately requires an Internet connection. No, I didn't become fluent from Duolingo. I also don't think it helped me learn new words. The examples just aren't that memorable! However, it does provide a place to practice and corrects your errors.
I tried another system of language learning[2], also involving daily practice, which has gotten me to the conversational level in the past 8 months. This has exceeded my expectations! I was never conversational even after 3 years of Spanish classes in high school. This takes far less time, and I think my memory for Spanish words is quite good nowadays! Some flash cards I've made to remember idioms have been useful in conversation, and certainly surprised folks I speak to in Spanish!
Many years ago I did a special diet called Bulletproof[3] on a whim after reading about it while surfing HN. I ended up sticking with it for almost a year. It's tough because you basically can't eat any sugar. I got really lean, which was interesting, but not a good look for me. The coffee recipe is excellent. It's a very interesting experience. It's obvious that but by far the most challenging. My body was very different by the end of it; drinking half of a PBR would make me very drunk and very sick.
[0] http://perigee.se/apps/seven/
[1] https://www.duolingo.com/
[2] https://fluent-forever.com/
[3] https://blog.bulletproof.com/start-the-bulletproof-diet/