Tracking time and energy
A simple colouring system
@juvoni's B.A.G.E.L system is a simple system for colouring.
I like how he uses red (danger) vs green (positive) as a simple colouring system. You could apply that also to calendar colouring. Green blocks for stuff you want to do more off and red blocks for stuff that has a negative impact on you. Makes analysing time and energy very easy.
Calendar UI's are one of the most interesting interfaces where we barely have seen innovation in the last 30 years.