Swarmkeeper
Hero
Many good points have already been made about how to make a combat more interesting. I like using a mix of enemies with varied abilities. For example, I ran an encounter with an Ice Maiden (Tome of Beasts), her Mammoth mount, and her pet Winter Wolf. The encounter started off on a calm note with the Ice Maiden chatting with the players before she charmed the gnome wizard and a round later blinded the rest of the party with a snowy blast. A blind party rolling with disadvantage added a layer of anxiety they hadn't experienced before - which was compounded by the fact that the cleric had just cast a Silence spell the previous round! The blinded half-orc paladin at one point, hearing no more enemies, role-played that he felt the fight was over and did nothing on his turn! The mammoth then destroyed the sorcerer (who immediately reincarnated as a dragonborn due to Wild Magic), but ultimately the characters prevailed. The combat had some real flavor that I could not even predict.
Another mixed-monster scenario is to introduce a third side who is attacking everyone. PCs are fighting against some bullywugs when a bunch of undead rise from the bog and attack anything close to them.
Throwing enemy spellcasters - perhaps with flavorful spells you as the DM have picked - into the mix always changes the playing field as well.
I think there are plenty of creative solutions that you can employ as the DM to keep combat fresh, challenging, and meaningful.
Another mixed-monster scenario is to introduce a third side who is attacking everyone. PCs are fighting against some bullywugs when a bunch of undead rise from the bog and attack anything close to them.
Throwing enemy spellcasters - perhaps with flavorful spells you as the DM have picked - into the mix always changes the playing field as well.
I think there are plenty of creative solutions that you can employ as the DM to keep combat fresh, challenging, and meaningful.