I just discovered these Mudpuppy Pouch Puzzles (via Amazon) and thought they'd be great for travel, whether via airplane, car, train or boat.
Each puzzle is designed for children ages 2 and up, and has just twelve pieces. The puzzles have a resealable pouch to keep all the pieces together.
I wouldn't recommend these puzzles for kids older than five, though, they wouldn't be challenging enough.
Puzzle themes include unicorns, ballerinas, farm and forest, a fire truck, under the ocean, zoo animals, puppies, pirates and more.
You can also shop for Mudpuppy pouch puzzles on eBay.