Dungeons and Dragons 5e is notable by the lack of movement that often occurs in combat. Player characters don’t tend to move during combat other than to rush towards an opponent (monks, fighters, and barbarians), whilst others move away (wizards and sorcerers). Clerics, bards, warlocks, and artificers move according to their subclass with melee builds moving forward and support and ranged attackers moving backwards.
In the event combat tends to be somewhat predictable. While predictability is perfectly find and a valid way to play, players and most certainly Dungeons Masters will get more out a combat event if the combat contains a few surprises. Here are some tactics to spice things up.
Adding creating levels in your combat encounters adds difficulty if played to the enemies strengths. Beholders, Dragons and Chimera, won’t line up to be punished by your barbarians and fighters when they have range attacks which they can rein down on characters. Humanoids too use height to their advantages. Think archers and ranged spellcasters in the gallery level protecting their queen.
Going low use water, fetid muds, quicksand, and lava as potential travel routes for creatures beneath the gaze of characters. There is nothing to stop a fire giant from diving into a pool of lava, holding their breath and after a short hidden swim rising to try and snatch a hapless character joining them in their fiery pool.
Responding to these threats will require characters to move use, use resources to engage or retreat successfully.
Throw in a dead magic zone
Used sparingly dead magic zones are incredibly tool for a protagonist who doesn’t have access to the magic held by the party. Dead magic renders may common tactics of parties inert. Spirit Guardians, Fireball, Polymorph, and magical weapons and items.
A creature such as a lycanthrope that is immune to non-magical weapon damage suddenly becomes an unassailable threat.
Use darkness to your advantage
By design many monsters have darkvision of 120ft compared to the players typical 60ft. Nothing threatens players more than a hail of arrows appearing out of the darkness from an enemy they cannot see. This particularly works well when using driders as even when the characters surge forward into the large cavern they discover their foe sits safe and high on the ceiling above.
Provide cover to spellcasters
Combat savvy creatures such as hobgoblins understand the battlefield advantage provided by spellcasters and they would be a strategic asset for them to protect. Assign key spellcasters with support troops who carry tower shields who will use their action and reaction to provide mobile cover to their spellcaster. When they are attacked by spells requiring a dexterity save both the spellcaster and shield carrier would take no damage on a successful save and only half damage on a failed save.
Use the environment to your advantage
Walls of force, eruptions of lava, obscuring smoke or steam are effective ways of transforming the battlefield into discrete zones. Elevators lifts, gantries cliffs, and tipped over vats of acid or molten metal can quickly transform a what was a simple location into something far more complex.
Combinations of trouble
Don’t limit yourself to a single option of harassing the party. But try not to use more than 2 at a time else you will struggle as a Dungeon Master to keep track of it all and risk upsetting your players with what could be perceived as excessively adversarial tactics from a combative Dungeon Master.