We have one that is a schwinn. It was $80 from Walmart. I collapses completely flat! My son LOVES it! It is a 2-seater, which does work out good because the extra space allows us to put a pillow in it and some toys/snacks.
We use it both around the neighborhood and out on trails. Our son just sits in it and hangs out, and usually ends up falling asleep on the second stretch of the ride (we generally ride 15 miles on the weekend days on a trail and 1-2 miles around the neighborhood during the week).
I love that my son is protected from the sun, bugs, rocks, etc. while he's in the trailer!