Although some of these have been mentioned elsewhere, I collect
-Old computers and peripherals
-Old video game consoles and games
-Handheld and other portables in the above categories
-Books
-Star Trek (TNG/DS9/VOY era) stuff
-Action figures, little toys, etc
-Puzzles (NOT jigsaw puzzles)
-Small sculptures and such
-Coins
-Knives
-(Mostly old) mechanical stuff I still use (watches, clocks, a typewriter, pens, etc...lots of metal)
-Stray domesticated animals
-Trees in pots (actually started as 3, but I save what I clip off and make more)
-Inexpensive (mostly small) musical instruments I don't play very well
-Science/history/whatever magazines (no current subscriptions)
-Mid-19th to early 20th century eyeglasses
-Old Android devices
-Containers
I used to have a lot more of most of the things listed above than I currently do. Most of that stuff has been lost in one way or another. I used to collect Harley Quinn stuff ("lost" it all), and am considering starting again. Some of the stuff in the above list is shared with other family members. Other stuff that is not really my thing includes a handful of old medical stuff.
I am not very hardcore with any of my collecting. Most of it isn't really collecting, and is probably actually just mild hoarding.