Here are some simple rules to make the combat a bit harder but not too difficult.
Simple rules for your session.
First off combat can take a bit too long so;
3 Combat encounters per 4 hours of play max
All of these should be close to hard or deadly. (see kobold fightclub)
All of them should be resolved somehow via non combat, (deception,persuasion,environment)
Simple rules for monsters
Remember that your Monsters in the Monster Manual are average and not exceptional versions of that monster. Create some variety in the HP as needed. Use these to increase their chance of hitting and putting some danger into your scenario. You don't have to use all of them at once by the way.
AC increase by +1
HP cut it in half, round down
Attack to hit bonus x2
Damage dice chain up one level and remove bonus damage (be wary of this)
All creatures have at least one recolored spells as abilities.
Become a bit more verbose in describing the player and monster actions.
Optional rules for players
Both inspiration and death saves are eliminated. Replaced by a luck score. Each character gets 1 luck at level 1, +1 luck at level 5 and 1 additional each 5 levels. Max of 5. Awarded by DM discretion for completion of a session or doing epic shit, being funny etc. Players burn this luck to come back from the dead DC 10 or higher for 1 hp or roll DC 15 for 1/2 hp. They can also use this to re-roll natural 1's or gain advantage. This gives the players a bit more power over their fate in the game. If things are getting too deadly start doling out more luck in the session.
Let's use the goblin as an example of how to use these rules.
Here is the normal Goblin stat block.
Let's change that up a bit, new values are;
AC of 16
HP of 4
Scimitar and Shortbow are now +8 to hit and do 1 D8 raw damage.
Let's give our friend at least one basic cantrip and recolor it as an ability. We'll use Vicious Mockery
Lets call it Goblin Slur and only allow them to use it once per day. Having combat open up with goblins cursing you and goading you into a fight is a bit more memorable than clubbing an HP bag to death.
Hopefully you find this helpful in making your monsters and combat a bit more memorable and deadly.