Teenagent

It's a 1994 VGA game by Polish developers, it's on GOG and runs on modern systems, and it can really crack you up at times.

It's divided into 3 parts, it's not that long (I hold the world record, shameless promotion), but it can be challenging for a new player. And I think these 3 guys would be able to figure out the puzzles, which means they won't be stuck and will finish the game. To me, it's an ideal Mostly Walking game.