Many of the Bike Fridays have a folding seat mast. But, I assume that adds a pound or two of weight. By removing the seat post, one gets a lighter frame.

I'm not sure why they don't fold the steering, but their solution is easy, secure, and strong.

Perhaps the easiest part to lose or misplace would be the pedals. Folding pedals are available, but I didn't think the ones I tried were very secure feeling. One might be able to get away with simply removing one pedal, and leaving the other side stick out.