My son did a lot of miles on his trailer-bike. I'm cheap, and just went with an InStep from Play It Again Sports. I used a C'Dale mountain bike with low-gearing as the tractor - and we did our local rails-to-trails in 10-20 mile increments, with one 32 mile jaunt. Had a lot of fun, and established our favorite places along the trail.
I got an adapter to mount a water bottle to the trailer-bike. We put it on the handlebars, so my son had easy access to cold water. I also noticed my son with feet up on the handlebars, or riding no hands / no feet. He also did the quick dismount on several occasions whenever he saw an anole or a skink running across the trail (he loves the lizards!). We're still working on the "let me know before you get off" thing.
We just went into a tandem bike this year. My son will be 8 on Friday, and was getting a little big for the InStep (but could definately still use it if we wanted to). We're having an easy time adjusting to the tandem.