For the geography-minded: Try to find the plates that are the farthest away from your home state, or use a map to group plates by region (finding all the plates in, say, the Southeast is easier than getting the whole country). Novar Cottage sells a small map with reusable license plate stickers for $3 (800/465-7230; www.novarcottage.com/statesplates.html).
For the math-minded: Look for sequences of numbers (first a 1, then a 2, and so on—you can go to 100), or race to be the first to add up the numbers on any given plate.
For the word-minded: See who can make up the most original phrase from the letters on a plate (HPR could be Harry Potter rules!); turn a series of personalized plates into poetry; or simply string together the alphabet, enlisting everyone in the search.
you can collect 'em, too
Most current U.S. plates (used ones) cost less than $10 and can be ordered via a number of Web sites (www.platesusa.com has a wide selection); they're also plentiful—both new and old—on eBay. Our favorite, the polar bear used by Canada's Northwest Territories ($11 at www.arcticharvest.com), is based on a seventh-grader's contest-winning design. Honk if you think kids should have a hand in America's plates, too.