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".
Tamagotchis are still fairly popular in Japan, though the ones made nowadays are incredibly different.
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.)
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 realize 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.