It took considerable convincing, but after my freshman year of high school my dad agreed to help me build a pizza oven in our backyard. Having never done any masonry before, this project was in completely new territory. Only being able to work consistently over summers, it took us over 4 years to complete, but the end result is a very high performing oven capable of cooking pizzas super quickly.
To date this is the largest scale project I've taken on. I learned a lot about masonry, but more importantly a ton about design and thinking through assembly from the very beginning.