My most transformational DIY victory was when I first installed a new exterior door into a house. I lived in that place for years and used it for carpentry practice, but I was always afraid of trying to change out the old 1970s windows and doors myself.
Finally I grew tired of the tiny little window in the master bedroom that didn't let in much light or views of the beautiful back yard gardens.. so I bought a giant set of glass/wood french doors to put there instead. Then I was afraid to install them, so I let them sit in the garage for a few months.
Eventually, I got a library book that explained the steps, and did the project - sawing a huge 6'x6' hole in the side of my house, adding the necessary framing, and then the door. It was surprisingly easy! It took only one long day of work, but it transformed the experience of living in that house forever.
Ever since then, I've been a window and door enthusiast, slicing new and better windows and doors into walls whenever I feel like it. It really transforms a house, and lets you bring in more light, solar heat, and nicer access to whatever side of the house is currently inaccessible.
Since most people are still afraid to do this (as I was originally), it gives you a big leg up on other homeowners when buying a fixer-upper for yourself at a discount. You can buy the place with good bones but awful windows and doors, while they won't be competing with you for it, since they imagine a cost of tens of thousands of dollars, while you just imagine a few Saturdays.