Years before Doom, John Romero wrote a small DOS platformer about a daring hero in a red cap, and Dangerous Dave is still easy to enjoy. Every level hides a golden trophy. Until you pick it up, the exit door stays shut, so the whole game is a hunt: find the trophy, survive the trip back.
The dangers are old-school and unforgiving: fire pits, deadly water, traps and monsters waiting right behind a jump. Dave has a memorable falling animation, which you will see often while you learn where the safe landing spots are. The jetpack is the one big helper, and it turns a long, nasty climb into a short flight.
It runs straight in the browser with no download, and platformer fans can follow it up with Super Mario Bros.