Yesterday's novelties turn into today's must-haves and tomorrow's collectibles — and ways to diversify investment portfolios — faster than ever before. CNBC.com reports that second-generation iPod players from Apple are now offered for sale on eBay for as much as $20,000, just two years after that series of product line was discontinued.

From duck decoys and hood ornaments to limited-edition sneakers and casks of whiskey, a host of other offbeat collectibles attract the eye of investors who crave tangible assets they can appreciate and that ... well, appreciate in value. Assembling a collection of rare items is as much a labor of love as an investment tactic and won't necessarily net owners significant returns. Then again, it might.

"In general, these are considered speculative types of investments," said Russ Robertson, a certified financial planner with WealthCrest Financial Services. "You can get great returns, but it comes with high risk, unless you really know what you're doing."

—By Sarah O'Brien and CNBC's Kenneth Kiesnoski
Updated and reposted 28 June 2016

Waterfowl decoys are hotly sought-after collectors' items, their value continually rising since the 1960s. In fact, a duck decoy sold in January 2014 at Sotheby's in New York for $767,000. It was a male eider created around 1900 by an unknown carver.

Collector Harry T. Williams, of Worton, Maryland, built his own "duck dynasty" of sorts. Eight out of three dozen duck decoys he bought for 50 cents apiece about four decades ago are the original core of his collection, he told CNBC in 2014. While Williams realized the investment potential of decoys, he did not make purchases for that reason. "I look for what I like," he said, adding that he focused on local carvers—that is, Maryland-based artists.

The market for collectible sneakers — such as the high-end pair from Los Angeles-based outfit Buscemi pictured here — is booming among young adults across the U.S. and abroad. We're not talking smelly gym shoes here; in 2015, Joe Diorio, owner of Brooklyn, New York-based shoe show SoleXChange, told CNBC he saw a pair of Nike Air Mags sell for $6,900. A limited number of Air Mags, based on sneakers Michael J. Fox wore in the "Back to the Future" movie trilogy, were released in 2011.

Diorio's company holds events in various cities, where attendees can buy and sell. This year's SoleXChange National Sneaker Convention will be held at the Meadowlands Expo Center in Secaucus, New Jersey on Aug. 27 and 28.

A subculture of automobile aficionados values anything related to cars—from antique chauffeurs' hats and licenses to spark plugs and classic hood ornaments. "Hood ornaments can go for a fortune," said Terry Kovel, co-author of "Kovels' Antiques and Collectibles Price Guide." Indeed, one was listed in 2014 on eBay for $199,999.

It's no joke: Trading in comic books, once the disposable domain of teenage boys, is growing in popularity as an investment option. As comics-related characters are increasingly spun off into fortune-founding TV, movie and video game franchises, the value of rare and mint-condition comic books has risen. In an extreme example, a single copy of the June 1938 edition of "Action Comics 1," considered the granddaddy of the superhero genre, sold for more than $2 million back in 2011.

As investing in comic books is a relatively recent phenomenon, there's no telling whether the segment will continue to grow or fizzle a bit, like the related animation-cel market has since its late '80s, early '90s boom.

They say it takes all kinds, and there's something for everyone. People who are mad about antique electric fans can find company in the Antique Fan Collectors Association, which in July will host the next installment of what it calls the Fan Fair convention in Louisville, Kentucky. According to the 500-member AFCA, the average fan collector has been collecting for 12 years and has 70 fans in his or her collection. Twenty members of the association have been collecting fans for three decades or more.

AFCA treasurer Dick Boswell says that most antique fans can sell for several hundred dollars each. Unique or rare examples can net owners quite a bit more; one such device, Boswell noted, fetched $13,200 in an eBay auction.

Collectors of clocks and watches—a tad more common than duck decoy aficinoados—have, of course, their own organization: the National Association of Watch and Clock Collectors, which owns and operates the National Watch & Clock Museum in Columbia, Pennsylvania.

Watch and clock collections can really bring in the big bucks and appreciate in value over ... time. Prices paid at auction for the most highly prized timepieces run into the millions of dollars. For example, the one-of-a-kind 18-karat "Henry Graves Supercomplication Watch" from Swiss luxury watchmaker Patek Philippe—designed for banker Henry Graves Jr. back in 1933—was auctioned at Sotheby's in late 1999 for an eye-watering $11 million plus.

Investing in whiskey is a relatively new pursuit. The market is smaller than that for wines, but that makes it an ideal one for trendsetters to get in on the ground floor. Whiskey collectors should focus on "single-barrel" whiskeys, where the supply is limited to the bottles from one cask—a supply that will diminish over time, according to Kenneth Waltzer, founder and president of California wealth-management firm Kenfield Capital Strategies. Avoid blended spirits; they're too consistent, and collectors want something unique. All whiskeys are more consistent and stable than wines, and so are the prices. Don't expect to toast any wild gains—or drown your sorrows over major losses—from your whiskey investment.

Then there's the not-so-great oddball investments. What's hot one day might be unappealing to collectors the next. Robot toys made in Japan after World War II skyrocketed in price for a few years—but then collector interest, and prices, dropped off.

"If you're investing a small part of [your portfolio] in a collectible and it gives you joy, that's great," said certified financial planner Clark Randall, founder and owner of Financial Enlightenment. "But I wouldn't count on that kind of investment funding your retirement."