A bunch of timeless toys (that never go out of style) come to mind, surviving in popularity with each passing decade -- jacks, slinkies, darts, dolls, action figures, miniature cars, frisbees. What's interesting is to see how they evolve over time, while still maintaining the core coolness that attracted children to love them in the first place.
If you look at the official Rubik's Cube website, for example, you will discover not just that familiar, brightly colored mind-twister, but also an amazing variety of other three-dimensional puzzle variants -- from crazy shapes and different sizes (the tiny to the ginormous) to apps and electronic Futuro cubes!
Another toy that brought me hours of pleasure as a kid was the View-Master. I used to have a collection of reels that depicted my favorite cartoons, movies, and television shows, some that showed landscapes, others that depicted animals (my favorites were the ones about dinosaurs!).
Now the View-Master is trending again, because it's capitalizing on the virtual reality fad. The new digital version is a bit more complicated than the older analog type (although that older style still exists for the purists). Now you download an app, place your smartphone in the new-fangled headgear, and look through the reels that come with "Experience Packs" (Space, Wildlife, Destinations, etc.) I haven't tried the new Virtual Reality View-Master yet, but thanks to first-hand knowledge from my own little offspring, I can attest to the fact that kids love VR.
Some critics might bemoan the need to stray too far from the simplicity of the originals, but these two examples show that kids of all ages can find joy in toys that have been around seemingly forever, and if their concept can be expanded into ever more variations while still remaining recognizable to its basic premise, then all is good. Play on!