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 1990s 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 stores 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. 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 toward nostalgic adults and are priced accordingly.
Any toys that resemble real-life firearms or weapons. Drug and/or gang wars of The Eighties and onward involved a lot of gunplay, and city kids who brandished realistic fake weapons risked being shot for real by criminals, police officers or both. To safeguard kids, toy guns now are brightly colored (black is disappearing), usually have a tell-all orange tip barrel, represent no commonly available firearms, and can't be easily painted to resemble real guns — they look more like fantasy/sci-fi weapons. Usually, their appearance is also out of proportion with real guns so they can be spotted at first glance. And cap guns have vanished due to the public's tendency to mistake their noise for real gunshots. Water-guns like the Super-Soakers, however, are still very popular.
Bratz dolls. When they first came out at the Turn of the Millennium, the fashion dolls were hugely popular in a way no prior Barbie rival had ever been, and very controversial due to their "slutty" appearance for characters who were supposed to be teenagers. Now thanks to said controversy dying out, several failed retools and spinoffs, its Live-Action Adaptation being considered one of the worst movies of all time, and the rise of doll lines such as Monster High that feature Loads and Loads of Characters with truly distinctive personalities and strong World Building, the Bratz franchise is struggling to stay afloat and is near dead.