Up till I was maybe 13 I'd collect my fallen out teeth. I keep them in a tin box. Haven't checked them in a couple years but I remember them holding up surprisingly well.
Not a hobby but when I was young I was also autistic about getting my hair cut. I'd keep chunks of my hair with each hair cut. Maybe this will mean something to me in the coming years since I'm already late stage balding.
Escapism - I self insert in fantasies I get heavily into. If I were to re-read Naruto I'd self-insert as Naruto. From conversations on image boards I don't think normal people disengage from reality to this extent, but maybe I was lied to. Nowadays since my brain has been deteriorating it has to either be extremely long so the lore encompasses a large part of my life (like Naruto), or it has to be really good. I miss the days I could watch a 24 episode anime series and self-insert from start to finish, in between episodes fantasizing my own plot developments. Now it has to be 80+ episodes, and usually only shit series make it to that point.
Another autistic thing I do every night while I go sleep is fantasize I'm holding someone. For the last six months that's been this girl and another girl from teen Jeopardy in November.