We have taken many road trips with our almost 4 year old twins (all in the 3 to 5 hour distance range). They've done well with that length trip and love the sights. We play "I Spy" games - like does anyone see something red, does anyone see something flying, etc. I'm thinking of an animal game - describe and they try to guess which one, etc. I also bring a variety of kids CDs or books on CD. I check ones out from the local library. Bring snacks and a small icechest with cold drinks for the day. Try to time it around their nap, so they'll sleep part of the way. Make sure you have something along to eat for yourself as they always seem to fall asleep right when I'm ready to stop for a lunch break and I don't want to wake them.
We are taking our first trip with them to Whistler in August, so let us know if you find any fun activities that you want to share in your feedback, too! We chose to stay at a place with a heated indoor and outdoor pool to have that option activity.
Also, see these sites - http://www.whistler4kids.com/parks.html
http://projects.ch2m.com/TrafficUpdate/travelling-sts.asp