I second a lot of the book suggestions (Magic Treehouse, Time Warp Trio, Horrible Histories, etc.). Here are a few more:

for younger kids:

Madeline books
Ms. Frizzle's Adventures in (name of country)
Good Times Travel Agency, a series of 4 books, each about a different coutry


for older kids:

Hachiko Waits, set in Japan
the Great Brain series, set in turn of the century Utah

I thought of some more:

Myth-o-mania series
Roman Mysteries series
You Wouldn't Want To... series

Also, we've enjoyed 2 series of activity books that come in the mail (so I suppose they wouldn't work well for people on the road). Both are produced by Highlights. One is Top Secret, which is similar to the Carmen San Diego computer games and videos: you are a secret agent in a country, trying to figure out which suspect stole what item from the country and where they hid it. The other, Which Way USA, has similar who/what/where questions to solve, but each activity book is about an American state.

I've loved re-reading this thread and have just ordered a couple of Japanese books off amazon. Does anyone have any suggestions of books set in South East Asia, Australia or New Zealand. Those countries seem to be rather under represented and it's hard to choose stuff on Amazon without recommendations!