Over the past few years I designed and built over 3,000 square feet of decks at varying elevations both connected to and disconnected from my house, including a series of platforms and stairs that drops 110 feet at a 50-degree angle down the hill in my backyard. I hand-mixed and dug all of the over 60 concrete footings and, given the positioning of a lot of the footings, would have to tie the wheelbarrow to two trees while I mixed so that it wouldn't roll down the hill. But I did myself what several contractors told me would cost between $60k - $80k to have built (after the standard "you want to build what?" response), which was more satisfying than anything I've ever done at my actual job. Plus, I think I built it better than someone else would have. I will say that the vast majority of it was built before we had a kid. No way I could find the time or have the permission to do that now.
One of the sections has an angled railing with aluminum frame and glass panels, which is the hardest thing I've ever had to build. I had to cut the aluminum frame pieces from 20-foot sections and, unlike working with wood, the compound mitre cuts on the aluminum had to be perfect to hold the glass so that it would fit in. It took me about 2 months of nights and weekends just for that railing.
For the most fun project, last summer I designed and built a drive-on double jet ski ramp for about 1/3 of the price of the pre-fab plastic ones. I've had several people ask me to either build one for them or sell them the design, but now that I've used it I want to tweak it a little this spring to make some improvements.