A recumbent bike with a very comfy seat. It's a 35 pound leather executive office chair connected to a 16" (little girl's) Princess bike re-welded into a recumbent (recliner bike) and using a piece of another donor bike frame. I built it in honor of "Bike to Work Day".
If you like fun/unusual home built bikes... check out my hobby site Woodenbikes.com
Step 1: Design your bike using a CAD system (Cardboard Aided Design)
Make a cardboard cutout of your lower leg (with foot and pedal), thigh, torso, and straight arm (to a distance 2" back from your wrist). Use it to look for good riding position and clearances for knees to bars, heels to wheels etc. Use the CAD system to layout the riding position, cranks, wheels etc with attention for locating your hands, shoulder, seatback angle, butt, knees and feet. Also look for ways to arrange a straight chain line (at least for the tight side) by raising the Bottom Bracket (BB)(main crank bearing).
For detailed instruction on designing a sweet handling recumbent visit www.bikesmithdesign.com