One of the cooler aspects about Fantastic Four was the vast amount of cameos from other Marvel Superheros throw in. The characters didn't speak, and only appeared on screen for a few seconds at a time. The first season featured one or two, but like X-Men, season two had them in nearly every episode.

The Origin Of The Fantastic Four, Part One

Submariner

Behold The Negative Zone

Iron Man Appears On T-Shirt

The Silver Surfer And The Return Of Galactus

Iron Man, War Machine and Ultimo

World Within Worlds

Throttle Appears In The Comic

To Battle The Living Planet

Various members of The Avengers and The New Warriors

Prey Of The Black Panther

Storm And Mjnari

When Calls Galactus

Franklin Storm Nightmare In Green Scarlet Spider, The X-Men and Juggernaut Behold A Distant Star Daredevil Hopelessly Impossible The Impossible Man morphs into Lawrence Limburger, The Hulk, a Ninja Turtle and Galactus Doomsday Namorita, Producer Larry Houston, Darkhawk and Justice, Speedball, Sabra, Nova and Galactus, The Avengers, The Avengers Mansion and The Blackbird.