One of my favorite habits is to write three positive things every day. I do this every evening and have been doing so for approximately 2800 days or ~8 years. I have rarely missed a day, so have written over 8,000 entries. It's very easy to do, because there are always good things happening and writing them down doesn't take more than a few minutes.
At the end of each year, I read through all the entries for that year. That takes 2-3 hours and gives an overview of the year. Obviously, it's a biased overview. Most negative events don't appear or they only appear because I write about some positive aspect. But I'm okay with that and it makes for an enjoyable and easy way to review a year.
In 2015, I spent a three months also writing "One Thing I Could Have Done Better Today". But that did not feel good at all. I already spend lots of attention and time on trying to improve things. I could see quickly that practicing finding flaws each evening would not have positive long term effects.
It's hard to know what the effect of this practice has been. Optimism and an ability to deal with adversity are definitely strengths of mine. And I suspect this practice has played some role in developing those.