There are a lot of ways to make combat more interesting in D&D. We've got some more advanced options.
Fighting is a major part of Dungeons & Dragons. There's nothing like a good combat encounter to change the course of a story. Taking down monsters and baddies gives players a chance to show off their characters' power. After a while, combat gets a little dull, especially at lower levels when characters haven't mastered certain key abilities yet. It's boring to use the same melee weapon until the other guy's dead. Players may be encouraged to strategize and work together by adjusting some of the combat mechanics. Also, DMs can give them additional options for taking down their enemies so they're more creative.
Switch the initiative order for a bit of fun.
You can change how creatures and players take turns taking turns in battle to make combat more engaging and create more strategy for players. When combat begins, players and opponents roll the initiative skill check, which determines the creature's order in combat based on their Dexterity. So the fastest and dexterous go first and the slower ones go last. Almost like a Dexterity save, this makes sense if combat surprises the party and they have to react. But when you get past the first round, reaction time isn't as important.
You can make combat more interesting by having a player turn and an enemy turn. Maybe a group initiative check could decide which person takes the turn. The party members would be able to react together and strategize together, using their abilities in the right order to defeat their enemies.
Let's add stamina to the equation.
A great way to make action economy even more challenging is to add a stamina ability. In Critical Role's one-shot about Elden Ring, they used an adjusted version of D&D. For weapon attacks and defending oneself from enemy att
acks, stamina had to be spent. During each round, stamina was replenished, so if a character used all of their stamina while attacking, none could be used for defense.
It is possible to use stamina as an additional effect for attacking in D&D, aligning it more with their combat system. A typical example would be a reaction to dodge or counterattack, or giving a standard attack a little more power, similar to a "heavy attack" in a video game. All of these things use stamina, which is calculated by adding class level to proficiency bonus plus constitution modifier. This would be balanced by the DM giving enemies the same power.
Adding more combat actions for players
An easier way to add some options players can use during combat is to add stamina instead of stamina to gain additional abilities. A list of additional combat actions is on pages 271-72 of the Dungeon Master's Guide. There are a bunch of them, like climbing onto another creature and disarming.
DMs can decide whether certain things, like throwing sand to blind a creature, are ranged attacks or Dexterity saves. There might also be options that require opposing ability checks, like throwing an enemy. If you do this, you might need to compare your Athletics or Acrobatics check to the target's. When you leave options like these available to your players and are flexible enough to cater to their needs, you can let them branch out and have fun in combat.
You've got to be careful with these things because there are subclasses that already have abilities that allow these changes, like Fighters and Battle Masters. The goal is not to remove the advantage that Fighters get from their action surges or Battle Masters' maneuvers. Incorporating new ideas into combat is just as important as keeping these abilities unique.