I've been keeping a bullet journal-style journal for the past month and a bit, and I've found it pretty useful. I don't write any "dear diary" style entries, but I do write down thoughts I have about game ideas, camera gear research that I might want to use/rent/buy, vacation ideas, and so forth, in the bullet journal style.
Here's an example empty page for a week - https://www.instagram.com/p/BPUIKR1Am3o/
I track some things daily:
* quality things
-- mood
-- energy levels
-- activity level
-- productivity
* tick-box things
-- did I wake up before 8am?
-- did I get to work before 10am?
-- did I hit my step goal?
-- did I do any additional exercise like a really long or strenuous walk?
-- did I do some work on a personal project? (taking photos, working on my game, looking at houses, whatever...)
-- did I record my weight and blood pressure?
-- did I follow my morning/evening routine? (pack/empty dishwasher, did I remember to take my pills, ...)
On a weekly basis, I tally up a few things from my activity trackers - how many days did I hit my sleep goal of 8 hours, what was my average weight/blood pressure, how many total steps did I do, and so forth. I also convert the daily quality things into the number of "good" days and "bad" days in that week on that scale - so maybe one very good mood day and two bad mood days (and the rest acceptable).
I then write down any non-technical-work tasks (we have a task tracker already at work for technical things, after all) I want to get done (either in the future, or on a particular day), or events that I need to remember, generally in the bullet journal style. These are things like:
* Collect building badge
* Write talk submission for conference
* Develop the iPhone photos from the weekend
* Remember that the new X book is out (today, on day X) and I should read it next
* On the 30th of January, remember to send out the invite to the event
Then every Sunday I make a new weekly page, migrate all the unfinished entries forward into the next week, and start again. I'll cross out things I realise that I don't care about any more, and move things that are further in the future than the upcoming week into the "future log". Pick two to three main things to keep in mind, schedule some work onto particular days, if appropriate, and the rest into a general weekly tasks list.
I also move things like vacation or camera gear or game idea one-liners onto a dedicated page - either a full page for the idea, or a page that has a list of those one-liners.