Prior to your trip, prepare a homemade "book" with a page for each day you will be gone. Write the day/date at the top of each page. As you travel, your child can use any sit-down time to fill in what you are doing that day, draw pictures of interesting things you saw, paste brochures, or get the autographs of anyone - from your waiter to the occasional celebrity! Your book will be just the thing to share with your friends when you return.
For long road trips with children, take a little spiral notebook and ahead of the trip write down names of all the cities you will be passing through. Then on the trip, as you get to each city, point it out to them and they can check it off their list. They get sort of a visual idea of how much more of the trip they have left and it gives them something to do every so often. On the return trip, they start at the bottom of the list and work backward. This worked really well with a six and eight year old on a trip from Michigan to Florida.