Where you write down thoughts/what you did/next steps. I use text files, but it's a little hard to look back on afterwards in a cohesive way. I'm wondering if people use other ways?
Plain text files are a great way to go. You should check out JRNL, https://maebert.github.io/jrnl/. It uses a single file and allows encryption and basically provides easy simple command line support for adding time-dated entries.
Text files and grep to look back. Then when a project starts to have some longevity I skim through the notes and make more formal documentation of some aspects of the work I'm doing.
I think I'll always stay with simple text files for projects that may or may not go anywhere.
I would recommend emacs org-mode. I've been using it for months now and became way more productive than before. You may want to take a look at this fine set-up. http://doc.norang.ca/org-mode.html
I'm not entirely sure why this was downvoted. I think this is a more than reasonable way of keeping track. And honestly, with git commit messages you can even keep track of the things you want to do next.
I disagree, I keep just about everything. Obviously the git is /the/ log for the code itself. But there's more to a project than code. I routinely take screenshots and just dump them into a "art dump" folder. It is nice to look back at ideas and features that went away, maybe for a "behind the scenes" look.
Git also doesn't keep ideas, brainstorm and discussion about them. I use Trello for this but it's just personal. But it lets you easily file things as "maybe some day" and "actually doing/done".