Context is the single most important factor for making combats varied in a simple combat system like AD&D 2nd edition's. There are few combat options because that's not where the variety is meant to lie: the variety is in why you fight.
Context includes the obvious terrain and PC goals, but also includes:
- Opponents' non-combat goals
- Opponents' purpose in fighting this fight
- Purpose of the fight for the PCs
- Whether there is an option to disengage
- Whether the combatants are fighting in self-defense or as aggressors
When the opponents have varied motives, they fight in varied ways. If their opponents are not fighting in varied ways, there's no reason for the PCs to bother either.
A fight in which the opponents' goal is to delay and distract the PCs while their superiors prepare an ambush is going to be a very different fight from one where the PCs' aim is to get through a cavern stealthily and the opponent's goal is to opportunistically capture something tasty for dinner. In the former the enemy is going to be striking hard initially then fighting defensively, and then breaking into flight after a while in order to draw the PCs in pursuit. In the latter, the enemy is going to strike quickly and withdraw quickly, with or without dinner. That variety in PC goals and enemy goals means the combat is going to shift and move, and force the PCs to respond, possibly without full knowledge of why the fight has shifted. These make for interesting choices!
Context begins long before the fight starts, and even before the fact of the fight is established. The PCs goals must be such that carelessness can compromise them (if not cause outright failure, which is less fun). The opponents goals must be more than fighting to the death in order for them to fight with variety.
An important tool 2e gives you to establish context is the Reaction table. Even for inimical, monstrous opponents, the Reaction table is valuable. Not every horse your PCs meet immediately charges them, knocks them down, and searches their pockets for sugar lumps and apples – so why should a monstrous carnivore behave that way? The Reaction table gives you as DM a hard kick in the habits, preventing you from setting up dull, predictable combats. A result of "hostile" is obvious, but what about "cautious"? Perhaps it means that the opponents have pressing matters to deal with elsewhere, and they'd really rather avoid losing time and able bodies in an unnecessary fight. Even a rabid pack of goblins might only warn the PCs away from "their" halls, and then retreat. In real life it's foolish to join every fight that an opponent offers, since then you are fighting on their terms – even goblins are not so stupid or bloodthirsty that every fight offered them is one they want to take. A lone predator is even less likely to join an even fight head-on, since any injury at all can kill them, off in the cold wilderness, even after a fight they win.
Even when the opponents do fight, make sure you use the morale rules – without them the NPCs are just cardboard cutouts to be knocked down. And besides, notice that the morale rules are reactive to the PCs' actions in the fight – not using morale robs your players of those tactical choices that they can use to force the enemy into a route faster, and giving them more useful fight options is the goal, right?
Let the Reaction table give you hints of when the NPCs might have better things to do, or want something other than the PCs' blood. Let the morale rules hint when the fight is done, in the enemy's eyes. Perhaps humanoids want allies against a more powerful aggressor? Perhaps the slavering creature is sore and tired and just wants to roar at them to go away? Perhaps the enemy will call a truce and parley when they see the fight is no longer worth fighting? And even when the PCs force the fight, your rolls for morale and on the Reaction table and can hint to you that the NPCs or monsters have different motives than just fighting to the death, which will inform your tactics. Again, varying your NPCs' and monsters' fighting tactics and purpose is the first step in making sure your PCs vary their fighting tactics and purpose.