For older kids, a trail-a-bike is a lot of fun. My daughter has been using hers since she was 3 1/2. The Wee-Hoo trailer seems particularly nice because you can put children in it that are too young to hold on (I've seen them in person, but never ridden with one.

I have a lot of experience with a trailer also. It's probably a cheaper route than an xtracycle (which does seem like fun).

Cheers,
Charles