For young students heading back to school each fall, few accessories proclaim their pop-culture tastes as conspicuously as a lunch box. Since their midcentury golden age and before, the metal carriers not only served as totes for tuna sandwiches and bruised bananas, but have been emblazoned with images of teen idols, characters from movies, TV shows, cartoons and more—from Batman to Snoopy to the Monkees.

Nostalgia for the colorful metal boxes has driven a strong collecting market. At the self-proclaimed largest lunch box museum in the world, in Columbus, Georgia, owner Allen Woodall has accumulated more than 1,300 metal boxes. He first got hooked when he bought a 1967 box featuring his childhood radio and comic-strip hero, the Green Hornet, in the 1980s. That find launched a lifelong collecting habit—one the 83-year-old shares with hundreds of visitors each year.

“Most people are searching for the box they might have had in school,” he says. “In many cases they don’t remember it until they actually see it in the museum, and then they light up.”

Kids have always needed to bring lunch to school, but plain old buckets and, later, paper bags reigned supreme for decades. Then the 1950s hit, and television shows fueled a golden age of licensing. According to lunchbox super-collector Thad Reece, more than 120 million metal lunch boxes in 450-plus designs were sold between 1950 and 1970 alone. And today a collector might pay up to 10,000 times the original price of a box for the most valuable examples.

Other school supplies, like textbooks, vintage educational technology, and even slide rules can catch collectors’ eyes. But for many, the humble lunch box commands the most interest, with more than 240 examples gathered in the Smithsonian National Museum of American History’s permanent collection. They’re “fabulous time capsules,” says Woodall. Here are some of the most desirable—and valuable:

This is the lunch accessory that started it all…but don’t call it a lunch box, says Mark Bellomo, a toy and pop-culture expert and author of Toys & Prices, which contains one of the most up-to-date lunch-box price guides. “It’s a metal lunch ‘kit’—a semantic difference perhaps, but a difference nonetheless,” says Bellomo.

Produced by Geuder, Paeschke & Frey, a tin toy manufacturer, the oval metal pail fastens together with a wire handle. Mickey and his “Steamboat Willy” friends frolic along the colorful sides. The kit retailed for 10 or 20 cents—a Depression-era price not all parents could afford. As a result, it’s rare indeed.

Kit or box, the container reflects a turning point in American lunch culture—the first time a licensed character was used on a lunch item. “I consider it the holy grail,” says Woodall. Today, it can fetch up to $2,000.

It took manufacturers another decade or so to realize that licensing plus lunch might equal big business. In 1950, Aladdin Industries, a lamp company turned insulated-vacuum-bottle vendor, struck gold when it produced the first real lunch box with a licensed character. This box had an innovation of its own: a matching thermos tucked inside.

The box was convenient, but its real appeal was the character it featured. William Boyd, who played Cassidy, was one of his era’s superstars. Aladdin sold more than 600,000 of the boxes in the first year, kicking off a cafeteria phenomenon. As a result, the boxes aren’t exactly rare, though a mint-condition example can sell for as much as $500.

Perhaps the most coveted lunch container, produced by Universal, features the Man of Steel. Bellomo ranks the box—which shows Superman battling robots and rescuing a damsel in distress—as the most valuable lunch box of all time, with a mint-condition specimen fetching as much as $13,000.

As few as 12 of this rare yellow lunch box are thought to exist today, which accounts for its high ranking on both Woodall’s and Bellomo’s lists of must-haves. The box could only be purchased with Top Value Stamps, a trading stamp given out by Kroger grocery stores in the Dayton, Ohio area. The more groceries you bought, the more stamps you’d accumulate—and you could use them to buy items like a yellow lunch box featuring the program’s whimsical plaid mascot.

Bellomo values a mint Toppie box at around $6,000. Heritage Auctions’ Eric Bradley adds that in a 2003 sale, a collector paid $2,717 for a far-from-mint specimen.

Merchandise that features the Fab Four will never go out of style, and this Aladdin lunch box was released just in time for full-blown Beatlemania. By the 1960s, lunch boxes were so ubiquitous that they could be used to boost a band’s popularity among preteens. Boxes like this one put the faces of heartthrobs and musical icons in front of kids on a daily basis.

The powder-blue box, featured in the Smithsonian’s collection, consistently ranks among the most expensive on the market, thanks to images of the boys in action, along with their faces and autographs. Values vary widely based on condition: According to Bradley, Heritage Auctions sold one for $750 in 2015, while Bellomo ranks its current mint value at around $1,820.

Lunch boxes helped bring TV from the home into the cafeteria and onto playgrounds and school buses. After all, the side of a lunch box almost mimics a TV set. This coveted Jetsons box, produced by Aladdin, re-created the cartoon future family’s daily routine on a domed box with matching thermos.

It’s “a great collectible box,” says Woodall, who notes that television themes dominated metal lunch boxes throughout their entire heyday. Mint versions can sell for as much as $1,700, says Bellomo.

America’s space race appeared as a common theme on lunch boxes, but catering to boys’ fascination with rockets, moon travel and astronauts had its risks. This Thermos-produced lunch box got its creators in hot water when it was discovered that the images of John Glenn inside the Mercury spacecraft had been stolen from the pages of National Geographic.

As a result, note Smithsonian experts, Thermos was given a cease-and-desist letter and had to stop producing the boxes. That means they’re pretty rare—a mint-condition box can cost up to $3,200. Even if there were more on the market, the “Orbit” boxes might still be popular. According to Woodall, “anything in space” still seduces collectors today.

Not all lunch boxes featured licensed characters—or themes aimed at boys. Boxes with names like “Debutante,” “Corsage,” “Junior Miss” and “Miss America” were designed to appeal to girls beginning in the 1950s. While not as collectible as those that feature popular TV shows, comic-book characters or dolls like Barbie, these aspirational lunch boxes are a reminder of the ways manufacturers perpetuated gender stereotypes.

Manufacturers like King Seeley Thermos Company faced stiff competition from vinyl lunch box makers and a glut of competitors, so they tried themes such as “Campus Queen” to help boost their profit. Since nobody owned the rights to this fictitious campus queen, King Seeley could make more money from each box. The box, which can be found at the Smithsonian, would cost about $330 in mint condition.

It’s unclear why metal lunch boxes fell out of favor, but in 1985 they died for good. Collectors like Woodall blame it on concerned mothers who moved to ban metal boxes for fear they were being used as deadly weapons; however, that story seems to be an urban legend.

The falling price of vinyl and plastic production through the 1980s is a likelier explanation, and in 1985, Thermos produced what is believed to be the last-ever metal lunch box with a licensed character, featuring Sylvester Stallone as an AK-47-toting Rambo. Though a mint-condition box will likely fetch only $380 today, it’s a reminder that even nostalgia can come with a bang.