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 there nowadays are nearly In Name Only. Virutal pet games have also found an audience with fans of 90's nostalgia.
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 and people of a certain age who buy them for either camp or nostalgia. 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.
Cartoon/Toyline tie-ins: These are now considered strictly a relic of The Eighties. The Transformers, G.I. Joe, Voltron, Masters of the Universe, and many others had popular cartoon shows that were arguably just advertisements for the toylines. Strict government regulations against child-targeted commercial advertising saw the end of this sort of marketing. Today, toys based on these older franchises are targeted towards nostalgic adults and are priced accordingly.
Any toys that resemble real life firearms or weapons. Toy guns are now ridiculously designed in bizzare colors (black is disappearing) and usually have the ubiquitous tell-all orange tip barrel. Additionally, they represent no commonly available firearms and look more like fantasy/sci-fi weapons. They are typically designed in a such outrageous ways that they can't be mistaken for any real gun or made of a plastic that can't be easily painted to resemble one. Usually, the appearance is out of proportion with real guns so they can be spotted at first glance. Cap guns have vanished due to the public's tendency to mistake them for real gunshots. (Water-guns like the Super-Soakers, however, are still very popular.)