Write down ideas... I'm very skeptical that this would work, because I have ideas about all kinds of topics all the time. My list would be so crowded and mixed that it would be hard to prioritize anything. At the end I would see on paper that there is too much stuff going on in my head, something that I already know...
The point is that you get it out of your head, and let it go, to focus on the task at hand. The fact that this physically accumulates outside of your head, is just a side effect, and just points out that you can't do everything, and so you have to let go of some task or idea, maybe for today, maybe forever.
I used to use GTD, but nowadays I mostly just use the "capture" part, a.k.a LET IT GO OUT OF MY SYSTEM DAMMIT. The remaining steps just sort themselves out as things are falling into very different buckets: a. very general life goals (non actionable by themselves due to their large scale, virtual nature, and can only be achieved by habit as well as internalising what I truly want, but certainly not with reminders), b. whatever I fancy doing when I happen to have some free time (for which I may go back to a quickly jotted draft of step 1, and since I'm in the mood for it, well, I just do it), c. very concrete errands and other mundane tasks.