If there's one thing all gearheads love, it's finding new uses for internal combustion engines. Some of these are practical, others are simply for fun, and still others are fantastic and provocative. While the methods, materials and purposes vary, putting gasoline power to anything that wasn't designed to have it requires skill, ingenuity and a heaping dose of creativity.
Making your own generator isn't only an exercise in engineering and creativity, it can save a lot of money and irritation. While purchasing a store-bought generator can be a very expensive proposition, home-built units can work just as well and cost thousands less. Most people who build them raid their local junkyard for the automotive alternators and inverter required. An average 2-horsepower engine and three alternators will provide about 3,000 watts of continuous power.
These have been around practically since the dawn of internal combustion engines, and are arguably one of the more practical projects one could embark upon with a small engine. Motorized bicycle kits are readily available online for between $150 and $400, depending on whether you like two-stroke or four-stroke engines.
Local laws vary, but any 49cc engine that lacks a manual transmission and is rated at less than 2 horsepower will be street legal in most states, with or without a driver's license. An average 49cc bike will do 30 to 35 mph (depending on gearing and rider), and get between 100 and 180 miles per gallon.
There is a practically endless variety of different ways to build home-built racers from a small engine. There are entire sanctioning bodies devoted to lawnmower racing, bar-stool racing, easy-chair racing and power-scooter racing. Although sanctioning bodies do not yet exist for them, a great number of people are converting electric-powered mobility scooters into hardcore racers. Of course, you could just build a go-kart, but consider one optimized for off-road performance.
If you're willing to trust a fabric parachute with what remains of your natural life, then paragliders are a cheap way to have more fun than anyone you know while simultaneously proving to them that you're completely insane. The basic components of paragliders are the wing-style parachute, a 200-250cc engine (strapped to your back), and a propeller. The hardest part about a paraglider isn't so much building it as it is getting off the ground, which often requires a stiff head-wind and some luck. Still, it beats driving in rush-hour traffic, and you can land one practically anywhere.