What are you into?
Learn to draw? Build/make something?
Learn to program?
Shoot videos with your phone and learn to edit (davinci resolve is a free editing software). Shoot whatever, little stories/vlogs/clips for friends.
Clean everything in your house, inside the fridge, the whole shower (above/below), dust the walls, go through your closet/drawers/bookshelfs/etc. The places you’d never otherwise pat attention to.
Move furniture in your home and reorganize the layout of your stuff.
Learn to cook, really dig into how to make sauces, properly cook steaks/fish/etc. Practice cutting veggies and perfect your knife skills.
I mean, there’s a million things to do.
Keep the box for your case and for your GPU. Pack those two items in their boxes. RAM/PSU/etc should be fine being stuck on the motherboard in the case. If you got a mechanical HDD, might be worth taking that out as well. And like someone else said, anything heavy should come out of the case. Be smart about the placement in the truck as well, put clothes/blankets/pillows under and around the case.