Any number of fad toys. Cabbage Patch Kids, Tamagotchi, Furby, Tickle-Me-Elmo, Beanie Babies, pogs ("tazos" in Mexico, Australia and other countries)... the list is ever-growing. Anyone who grew up at the time that any one of these were popular has probably witnessed the popularity arc go from "niche item that only a few people have heard of" to "waiting in line for half a day just to get one, then seeing people fight in the store over the remaining stock" to "finding a bunch of well-worn ones for 50 cents at Goodwill".
For the fogies in the crowd: Pet rocks. Mood rings. Lava lamps. (Although that last one has never quite gone entirely away, but is now mostly the venue of young kids. A lava lamp seen in a movie or TV show is an indicator that its owner smokes a lot of pot.)
For people of a certain age, we've seen our following childhood toys go through this: Cabbage Patch Kids (though they're making a valiant attempt at a resurgence), Teddy Ruxpin, Simon, pocket games that play only one game (such as Pac-Man or Centipede), Pound Puppies, the Snoopy snow cone maker, Strawberry Shortcake, and Rainbow Brite (though those last three are making something of a comeback).
Home chemistry sets and other science kits, thanks to a combination of perceived dangers to life and limb and the Wars on Terror and Drugs. The Gabriel and Skillcraft sets were highly acclaimed for being equipped with everything a budding young chemist needs: scaled down chemical glassware (real Pyrex), apparatus, and a host of chemicals used by real chemists. As a bonus, you could order from their catalogs for even more items. Due to alleged concerns about safety and liability, test tubes and beakers are now plastic, there are no alcohol burners, chemicals are now mostly limited to vinegar, table salt, table sugar, and other "safe" household items that are expected to already be on hand, and experiments are limited to simple, boring reactions such as color changes.
On top of that, there is a fear among police and various "public safety" groups and advocates that chemistry sets allegedly teach kids to make explosives and methamphetamines. (Breaking Bad has likely done little to help with the latter image.) Thanks to this, there have been a number of highly publicized police raids into the homes of what were nothing more than chemistry hobbyists. Some states, such as Texas, have even gone as far as making possession of any chemistry glassware illegal without a government permit.
Hobby telescopes: These are often sold in toy stores, department and novelty stores and they are usually priced anywhere from $49.99 to $99.99. With a little research, any budding amateur astronomer will instantly realise that the only good telescope is a telescope that you purchase from a dedicated and reputable science vendor. They are more expensive but if one is serious about the hobby, it's worth the investment.