I'm not a sit-on-the-beach person, so I'm usually reading for relatively short periods of time, or when I'm very tired. Obviously, any book has to be interesting and hold my attention, but vacation reads mustn't have plots that are so complex that I have to go back a chapter or two to remember what's going on.

Rereading old favorites also works. Agatha Christie, Mary Roberts Rinehart, Ngaio Marsh all wrote mysteries that were enjoyable, non-demanding reads. Additionally, many of these involve excursions or vacations, which kind of doubles the fun!
If you are a beach reader, perhaps a book that takes place in the winter will keep you cool, such as Jenny Milchman's Cover of Snow.
If your vacation involves a lot of driving, give audio-books a try. I recently listened to The Boyfriend by Thomas Perry going back and forth from work, and it was wonderful.
Apart from mysteries, travelogues are a lot of fun to listen to on trips, again, "doubling the fun". Some years ago, my husband and I listened to Bill Bryson's tales of his tour of /Australia, In a Sunburned Country and laughed for a good portion of the drive. This year, we're going to try Peter Mayle's Provence books.
I'd love to hear some readers' recommendations for vacation reading.