Thanks! I did discover some things that made a huge difference in my son's enjoyment; First, it was important that he participate in the decision-making. When we got to a tricky intersection, I'd make sure that we both agreed which way to go. As it turned out, he caught some errors that I would have made, so it was a good idea all around.

Second - and this might be very dependent on the child - Ben is the type of kid who would ask every half mile "How far have we gone now?" Not because he was bored, but out of simple curiosity, and a desire to beat the number of miles we did the previous day. A cheap bike computer was a great investment for him. Though I did have to scold him a few times when he spent more time looking at the computer instead of the road. Combined with small-scale goals ( "Let's get to Rome by 3:00PM", for example ), it kept his interest high. And I'd always have a large-scale goal at the end as well, like a promise to get ice cream after we got to the hotel.

Third, he's the type of kid who will NOT admit when he's about to bonk. I had to be very careful about keeping an eye on him and and making sure he was eating and drinking enough. Essentially, we had a picnic on wheels. ;-) Just looking for signs that he was petering out, like not talking as much, etc. helped a lot to make sure we both ended up happy at the end of the day.

Finally, just have fun. Don't begrudge lots of stops to look at snakes, flowers, etc. You might end up being on the bikes for six hours instead of three, but it makes a huge difference in how much enjoyment everyone gets from the trip.