In The Creative Habit Twyla Tharp talks about "starting a box" to keep track of all the creative ideas that come on a project.
I've used git to just start tracking ideas in digital form. A brand new git repo is my digital box. Usually, the ideas start out as a whiteboard diagram or scratching on paper, which I then scan to PDF and put in the folder. The organization varies, but chronological file naming has worked for me, as has thematic subfolder organization.
Being able to just capture the work frees me up to try new things and not worry about whether that stuff I just did is safe or whether I can get back to it if I try something new out.
Git does such great cross-file compression that even after several revisions across binary files, the resulting repo sizes are surprisingly small.