I used to use FreeMind [0], a mind-mapping tool that I really like, to keep track of projects, ideas, and things to follow up on. I eventually noticed that the pretty pictures could be represented, without loss of information, by a simple outline in a text file. Reading 'Getting Things Done' [1] got me to think in terms of a 'Current Task' list, which I now keep as a text file, backed up on Dropbox. Anything I need to do, or want to follow up on, or want to dream about in a practical way, gets jotted down in the file, with the most urgent matters at the top, Some really interesting stuff drifts down in the file as time passes.
As I do stuff, things move from the Current Tasks file to a Logbook file. Beyond 'accomplished todo's, that file gets stuffed with, for example, interesting quotes I run into, command lines I think I'll need again, and a record of what I've been up to. When I need to put together a status report, or figure out for myself where my time went, or recall how to do something intricate that I haven't done in six months, I turn to my Logbook.
With these two pieces, I feel like I've got a handle on what I want to do, and a record of what I've done. Now, if I just could find the time to follow through...
[0] http://freemind.sourceforge.net/wiki/index.php/Main_Page
[1] https://en.wikipedia.org/wiki/Getting_Things_Done