I recently decided to upgrade my Dahon Boardwalk S1 which I use as a commuter bike. Got some great tips from this forum and another blogger and figured I'd pay it forward.

My upgrades including replacing the rear coaster brake hub rear-wheel with a 110mm hub free-wheel. I installed a 16t sprocket on the freewheel. This really lightened up the bike and makes for a zippy ride.

Because there was no longer a coaster brake, I installed a caliper brake, by mounting it to the frame where the fender was originally mounted. I had to use a long reach caliper and install it upside down. The Tektro R559 (make sure u get the hex bolt version as opposed to the recessed nut)

In addition I had previously installed some shimano v-brakes on the front wheel. I kept them on, but with the caliper brake lever it is a little spongy. So, I may replace the v-brake with a caliper in the future or install a travel agent.

I replaced the crank set with a black origin 8 46t 170mm length crank set. (Amazon.com : Origin8 Pro Pulsion Single Speed Crankset - 170mm x 46, Square Taper, Black : Bike Cranksets And Accessories : Sports & Outdoors)

I had previously replaced the stock pedals with MKS FD-7 Folding pedals found here, Pedals and Shocks.
These pedals spin great and provide good grip, even in my dress shoes.

I installed Red Kenda 20 X 1.90 tires on the wheels, it gives the bike great stability, although it might slow it down some, I cannot feel the difference.

I also replaced the handlepost with a handlepost I took off a salvaged Dahon Boadwalk S7 (circa 2003). I painted it black, and added a litepro handlebar extender from amazon to give me a little more reach. I also put red handlebar grips. The added reach makes the ride feel much more similar to a full-size bike. And, as depicted below it still folds just like before if you release the quick-lever and point the handlebar down.

Lastly I replaced the stem with a Rockbros 33.9mm stem and a spider web seat, to drop some more weight.

This bike is great, it rides like a zippy single speed and takes potholes like a champ. Its also much lighter with the upgrades. Weighs in at about 25 lbs. Plus it folds up to a small package so that I can fit it on the train. It fits on the floor next to me on the new and old NJtransit trains and fits on the overhead luggage rack of the old trains and in the racks in the middle of the new trains.