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. A lava lamp seen in a movie or TV show is an indicator that its owner smokes a lot of pot.)
Once upon a time, any Fantasy,Science Fiction, or Action Adventure movie or TV series would get a short lived toyline. Planet of the Apes, Six Million Dollar Man, Star Wars, Star Trek, Raiders of the Lost Ark,Space1999, Battlestar Galactica,Buck Rogers in the 25th Century, The A-Team, and Knight Rider all had toylines. Even seemingly unlikely candidates such as Ralph Bakshi's The Lord of the Rings, the R-Rated Alien, BladeRunner, and Rambo or even the PG-13 rated 1984 Dune had toys produced. Most of these are still collectible today. What is worth remembering at the time is that these toys were targeted towards children and not towards adult geeks. This makes it amazing that they created toylines based on films that children were not likely allowed to watch, such as the aforementioned R-rated films. Don't look for any toylines based on most of todays genre shows or films. Today's trend is towards more naturalistic settings and characters in genre films and series. A good number of them are strictly adult level and not suitable for viewing by children. At the very least, children may be bored by the lack of action. The current trend in genre shows is slow paced episodic stories that focus on characters and their relationsips. Making action figures for such shows is somewhat absurd.
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.