A Journal A Day
I keep a journal and I contribute to it every day.
It may not be the lengthiest entry, and sometimes I'm downright lazy.
What I really enjoy is looking back at the past. It's more of a diary than a review of my general mental health, so it's great to see what we did 'on this day in history'.
I started keeping a journal over 4 years ago, which started when I used notes in Evernote. A new note for each day, with a specific naming convention to keep them together.
Eventually, I moved to Day One, a dedicated app for journaling. I've been using it every since, and it's great.
I've added photos and even audio recordings which, when then time comes, pop-up to show me what we did.
I may not use it to record my current state-of-mind - I'm rarely down in the dumps, but it has helped me to blow off some steam in certain situations.