L-theanine taken with coffee makes me less jittery or anxious while also getting the benefits of caffeine. This is a cheap intervention with no observable downside.
Melatonin I've used to regulate my sleep and try to advance my sleep phase in the winter months when I have issues with it. This in turn improves my cognitive function, although the more important thing is just going to bed at the same time every day and getting enough hours in.
I've tried microdosing psilocybin too - it's highly effective. I really feel better in every way when taking this - focused, calmer, more emotional (mostly a good thing). You can't do it everyday though and I am hesitant to take it frequently. Now I mostly take it to help me get out of a slump.