As others have said, installing laminate/engineered flooring is crazy easy and offers a big “bang for the buck” improvement. I haven’t physically done it (so others may correct me), but installing tongue in groove hardwood flooring seems to be nearly as easy too.
I don’t know what you mean by easy, but most concrete work isn’t complex, so it’s not too difficult to get it to look good. It’s very physically demanding though.
Replacing light fixtures/outlets/switches.
Repairing tile grout
Installing ceiling fans
A hell of a lot of car repairs are pretty easy and can save you a TON of money. Most can be done with a simple ratchet wrench and socket set. Youtube how to replace the part you need on your cars model year to see the difficulty level.
More difficult projects (I’m not saying these should be hired out, just that you may need special tools or expertise):
Replacing windows. You don’t know the conditions of the framing, insulation, capping, etc until you rip the old one out. If you’re going to do this, be prepared to completely reframe and recap the whole window.
Hanging new interior doors. You need to be fairly precise when cutting to size and when cutting out the mortises(indents) for the hinges and lock/handle set.
Replacing “sections” of walls, rooms, etc. I put this for a couple of reasons, mostly because when you rip out one part of a wall or room, you never know what’s behind what you’re ripping out. There are likely to be unforeseen complications. The other reason is project creep. If you decide to replace a bathtub with a shower stall, you may decide that you “need” new tile flooring; then a new sink; then a new cabinet; etc, etc, etc.