There are DIY kits out there that I think are pretty cool, like making your own espresso
https://www.diypresso.com/product/diypresso-one/
I've seen others like making your own beer, or making your own speakers. Or like your own mini arcade cabinet on your desk. Do you have a favorite that you've bought and built?
I also loved building my Prusa printer. That took much longer to build (like probably 5 hours for me). But it was really cool to learn all the parts and feel really comfortable with how it works.
[1] https://frame.work
[2] https://www.prusa3d.com/category/3d-printers/