A thread on the Microlite20 forum got me thinking about making undead, especially zombies and skeletons, more difficult opponents, but without raising their hit dice or hit points, and without adding much complexity. The idea is that zombies, at least in zombie movies, keep coming at you and ignore wounds that would kill a living creature; they stop when decapitated or shot in the head. Fire may also destroy them. Similarly, skeletons sometimes keep attacking until smashed to bits.
My idea: for ordinary attacks, don't mark cumulative damage against hit points. Instead, roll the zombie/skeleton's hit dice; if you do more damage in a single hit than the number rolled, the zombie/skeleton drops, stunned, and you have a chance to finish it off before it gets back up. Aiming at the head does a critical injury instead of hp damage; once a head injury is higher than the zombie's hit points, the zombie is truly dead. Likewise, fire or acid work as normal against zombies, and attacks with blunt, smashing weapons work as normal against skeletons; other non-weapon attacks may be viable as well.
This can be adapted to other undead or to constructs. Ghosts might ignore non-magical physical attacks, compare non-cumulative damage on magical physical attacks to a hit dice roll for a temporary stun, and be destroyed by appropriate non-physical attacks that do cumulative damage more than their hit points, as well as by destruction of their physical remains. A lich might treat all attacks as a stun and attacks aimed at an amulet, canopic jar or periapt as critical injuries. This allows the GM to create custom monsters that can only be truly destroyed in specific ways, leaving the players to guess or research those ways.