I used a 3 speed, gear-hub, girls bike, but you could probably adapt the simple design to work with any bike. The end result is pretty goofy looking but very effective at hauling people and cargo. You can find a bike like this for $15 at goodwill and should plan on spending $10 on the bolts and wood.
Step 1: Tools and materials
1" paddle bit
1/2" twist drill bit
x4 lag bolts, nuts, and washers
x2 6 foot long 2x4s
( you may also want a spokeshave/ sanding block to shape the peddling area and a coping/ jigsaw to cut out the front brake area )