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- Gardening: it requires little things but regular attention, and you get food and pretty flowers out of it. If you can't do it at home you can try guerilla gardening somewhere in your neighborhood.
- Astronomy/radio astronomy: nothing more regular and scheduled than the stars.
- Cycling (if you live somewhere where it's safe to cycle)
- Swimming/Trekking
- Do you know the youtube channel Save it for parts? Maybe it can give you some inspiration, he also has ADHD and he's always jumping between different projects.