Carbon monoxide for everyone!
Also known at the Sociable, the side-by-side tandem bike has actually been around since the 19th century. Commonly used for courting couples or newlyweds, the bicycle is seeing a resurgence with hackers and DIY enthusiasts. One guide by Instructables user carlitos is a simple weld job consisting of nothing more than an extra seat, pedals, crank and some tubing. I can't think of a more perfect wedding gift.