I have built a number of heavy duty bicycle trailers. Here are some rather old and poor pages on the first and second cycle trailers I've built, and some loads I've carried. The most recent one I've built is my third trailer, now on its second hitch design.
If cycling leaves your arms wasting away why not try a Waterrower? Race online with others around the world at the still-in-beta but useable We-Row. I don't use it as much as I should nowadays...
I've built some rather fine-sounding single-driver loudspeakers to a design by Bob Brines.
And built a cool valve amplifier from a kit by World Audio Design, now trading as World Designs. When I bought my kit the prices were a LOT lower!