Before my son could sit up, we just put the whole car seat into the trailer and belted it in (my trailer is an older Burley that doesn't have the molded seats - it may not work with yours). Worked wonderfully! He could ride forward and it would put him to sleep in no time. Just be careful not to ride over rough terrain or go slowly if you have to.

I used to also put him in a little bouncy chair with some toys while I spun on my trainer. He was fascinated with watching the wheels go round. Course, now that he's 3, he wants to ride in the street like Mommy :-(