Plan ahead and use some creativity - and the trip won't be as bad as you think. Here is a list I compiled based upon some of what works for my son when we travel. These don't always work all the time, but usually they're helpful.
1. Buy them an inexpensive digital camera and have them practice using it before you leave so they don't get frustrated when they get there. When you get to the destination, let them take their own pictures. This doubles as a souvenir and an activity to keep them busy. Also, it's amazing to see the world through the eyes of a child. They usually see what we as adults miss.
2. Let them read books about your destination before you leave. This way they get interested in the place and might enjoy some of the mainstream attractions more if they saw them in a book and got excited about them first.
3. Let them draw a picture of whatever you just saw or toured.
4. Let them write about what they just saw - if they're old enough.
"Remember that happiness is a way of travel - not a destination." -Roy M. Goodman