pen and paper.

i have a graph paper notebook for "real" stuff like designs and plans and a little top spiral notebook that lives in my back pocket for quick reminders, notes and ideas.

periodically throughout the day i'll look at the little notebook and knock out stuff on it or move them to the big one.

the "rite in the rain" brand has waterproof notebooks you can use for when you need to write shit down in tech-averse climates.