Having a physical planner and writing in it using only pencil helps me a lot to budget energy. I plan out all energy consuming activities ahead for each week. I plan to do 5 stressful tasks each day, but what counts as stressful changes constantly, which is why I write in pencil. If I don't have the energy for something on one day I will erase and reschedule it and write something less demanding as a task, otherwise I always have to do what's on the list. I am not allowed to do more than 5 stressful things even on a good day, because then I usually overwork myself, and I have to plan in recovery time after an especially demanding day. Also, I break down big tasks beforehand, so I can be as specific as possible with my task setting (Goblintools can help with that). I use a physical planner because it gets me off of my phone and because I can remember stuff better when I write it down by hand, and I use pencil so I don't have to cross out unfinished tasks and be reminded of "failing" when looking back.

So yeah, TLDR: an analog planner with strict rules on how to budget energy and some flexibility to acommodate fluctuating energy levels