Lair Actions. Most monsters only get them if they're very powerful, but you could have Lair Actions for all kinds of battles where you enter an area that could have traps, random allies showing up, or other exciting events. For example, in a recent session, the players were beset upon by Harpies who had made an aerie on the top level of a ruined tower. The harpies hurled torches and oil down at the players. The tower was also old and not completely stable, so there was a risk of debris or loose stones if you tried to climb it (which some characters had no choice about, if they were subject to a luring song). To add to the chaos, I added the following:
At Initiative Count 20 each turn, one of the following occurs (roll d4):
1- a loose stone is dislodged, striking a random enemy (edit: whenever I say enemy, I mean an enemy of the Harpies), who takes 10 (4d4) bludgeoning damage, DC 13 Dex save for half damage. If this save is failed by 5 or more, the target is knocked unconscious until initiative count 20 (they wake up if they take damage or an action is used to rouse them).
2- the floor proves to be unstable, and a random enemy suddenly finds their foot falls through the floor and is trapped! They are Restrained until they use an Action to free themselves.
3- a discarded bottle of oil suddenly ignites due to being exposed to a thrown torch. A 5' radius area of burning oil appears. Creatures who start their turns or enter this area take 5 (2d4) fire damage!
4- a broken glass under a random (non-flying) character's foot breaks, impaling them foot with glass shards. They take 2 (d4) piercing damage and their speed is halved until a DC 10 Medicine check is made as an action to remove the glass.