Sofa bed in motion - got the idea from someone on Craigslist offering a broken futon frame (free), and suggesting people to make a bike trailer out of it. Why not, it has been so fun that we have decided to build more variations of the theme; see more photos on facebook.
Python trike is probably the simplest design, and quite compact. For exercising purpose, all 3 riders get to pedal. All have wheels have coaster brakes (kids bike), so synchronized braking is required. The seat layout is very pretty good for communication, without head banging...
The halves of the futon are welded together, with re-enforcement at the front pivot and rear handle. The frame ate up a lot of mig wire. Being the first python built, without a jig and only eyeballed, the 65-deg pivot came out tilted to the right, but surprisingly still ridable thanks to the self-centering effect. A picnic umbrella mount was added later, for rainy days or just a picnic in the park.
Quite heavy, probably 100+ lbs, but reaching 8-10mph is no problem, provided all riders pedal. Going downhill is a bit scary at first, but quite stable without pedaling. It clocked 18mph on a mild slope. With coaster brakes, we probably don't want to go any faster than that...
With just 3 20in wheels @ 35psi, we were surprised at the load capacity, at the limit ~650lbs (5 people), so 500lbs (3 or 4 people) is fine if more than 1 person pedals.
The trike is a little unstable for 2 riders (unbalanced) depending on their weight. DirkS suggested to reverse the configuration to a tadpole somehow, perhaps by converting coaster-brake to fixed-gear hubs.
Held a golf umbrella in the rain several times, which can cover most of 1 rider, and is not too bad without strong wind. The picnic umbrella can covers most of the riders, and has not been tested in strong wind yet...
First question new riders ask is "where is the handlebar for steering?". Once in motion, they would fine all kinds of use with their freed arms, crunching, crossing, waving, or texting...