Well, the point of quick, rules-light combat is not to make combat interesting but to get it over quickly and get back to the interesting parts. Making combat more deadly (or otherwise expensive) so that your players do their best to avoid it can keep it to a minimum too. (And get them looking for more interesting solutions.) Using morale rules (even if they are just a reminder to yourself and you always decide rather than roll) can at least make how combat ends more interesting. That said...

First suggestion: Environment. Create interesting places to have combats. Things that the players, opponents, and tactics and interact with. Even if you had not planned for a combat in an area, try to think of things that might be there that could make things interesting. Hazards, obstacles, broken ground, slick ground, pits/cliffs, multi-level, choke points, areas with cover, open areas with no cover, chandeliers to swing on, etc.

(You can also enlist the aid of the players. Encourage them to ask leading questions. “Is there a chandelier here I could use to swing across the room and behind the enemy?” There is now!)

Second suggestion: Have other things than combat going on during the combat.

Maybe there is a fire raging at the same time. The players or the opponents have to avoid it. Maybe they can use it tactically. Maybe the players or their opponents have an interest in keeping it going or stopping it.

Maybe the players or their opponents are trying to complete some procedure. (Ritual, repair, sabotage, etc.) How much time does each combatant devote to fighting versus completing or interrupting the procedure?

Maybe the combat is happening in a crowd of bystanders that neither side wants to hurt.

Third: Interesting enemies.

People like to rag on the rust monster for being metagamey, but it does make for a more interesting combat where the usual tactics don’t work. The thing is—in the end—it is the playing of the game that we want to make more interesting. Metagaming is doing that directly, and you are smart enough to rationalize a veneer over it. The better the rationalization, the more “immersive” it is, but never let a thin rationalization trump a fun game. (Just my opinion. YMMV of course.)

Look through monster books, especially for older games, for monsters that have their own specific rules or exceptions to the rules. (Spell effects can also be inspirational.) Unusual forms of movement. Area of effect abilities. Unusual vulnerabilities. Attacks that don’t do conventional damage.

Hope that helps.