There was a game design mental exercise I used to do quite regularly. And I did this mental exercise on-again, off-again for about eight months total.
I would spend two or three hours drawing simple graphic assets, select a palette of limited colours, and a font, and a dozen sound effects.
I would then try to make an interesting game, and actually implement the game in code, out of only those assets I had available.
The mental exercise had a tendency to strongly focus the mind on what was important.
Sometimes you wound up with something interesting.
Sometimes you wound up with nothing at all.
But you always wound up with an interesting lesson in game design.