Anytime a kid asks what they should build people start spouting off suggestimg all sorts of machine work. It's like they dont remember being a teenager trying to build a car,pay car insurance,buy condoms(awfully optimistic) and take chicks out on a 25 hour a week paycheck at minimum wage.
The best suggestion for a kid is always whatever you can put together with little to no cash outlay.
Quality machine work isn't free.