My first thought was Lite Brite, but I guess that one is actually still around. I picked three out of all the ones I could think of. The first is this.
this is a 45 rpm adapter, or a "spider". It allowed you to play a 45 record on a 33 record player. It fit inside the 45 to make it stackable (so you could play one right after the other) and to hold it in place. This ingenious device was invaluable to those with a 45 collection. My second choice would be Viewmaster
which I actually still have here. You put the disc into the viewmaster, and look at a picture and read a bit of a story, then click to the next one, sometimes changing discs until the story is complete. Hours of fun. My third choice is the Fisher Price movie viewer.
each cartridge contained a short movie or cartoon. You put the cartridge in the projector, look through the eyepiece and rotate the crank on the side. You can vary the speed so it goes faster or slower. My favorites were a 3 stooges one, and a Goofy, Mickey Mouse ghost one. @rambo @missy