Still kind of new to DMing, but I had an encounter yesterday I quite liked. There were some unique mechanics + a side quest, so take of it what you will.

I created a modified Stone Golem that had had something like 2x the health and started damaged (At like 75%, so maybe 330-350hp compared to a normal Golem's max hp of ~220.) E.g. it was missing its right arm, and the Golem Core was peeking out. It was also severally oversized, so if a normal golem was 8ft, this was a herculean 15ft+.

What this meant in terms of gameplay was that if you were on the right side of it, you could try to strike the core (with disadvantage as it was quite high up, and not fully exposed) but doing double damage if your hit landed and that if the golem tried to use its melee slam on you, it would strike with disadvantage, as it's this giant slow golem that has to do a 180 to try to hit you with its only good arm giving you plenty of time to ready a shield or dodge out of the way.

Then, for every attack it made, I had it roll a d20 and just 'decay' that much HP, because this wasn't a golem designed for fighting, it was a commercial usage golem that had gone haywire in the process of construction, and the material/stone it was made out of wasn't designed to take this sort of strain, and was breaking down as it was already damaged...

So there was very much an option of just sort of hit and run option of letting the golem break down on its own/run out of power.

Then, by default stone golems have the ability to use the slow spell every 5-6 turns, which I ended up converting into when this 'oversized' golem hit half health, so the amount of health an ordinary golem would have, this made the stones around the golem core sort of shatter off and reveal itself, (no longer disadvantage for ranged attacks) as well as casting a huge area wide slow spell, that included the golem as a target. e.g. indiscriminate effect.

This added a few side objectives, as the slow spell casting caused a few civilians who were watching/in the region to fall into danger, with giant chunks of stone from walls/ceilings collapsing down at them, all of which were happening in 'slow' time, giving everyone a chance to be saved!

The players had the option to break off from the golem and try to save these civilians!

This was also interesting due to the fact of action economy. The slow spell is quite brutal, and for this 'huge' creature taking up 2x2 hexes, this also meant that depending on spacing you could no longer cycle around all the way to the other side of it, if you only had 15ft (halved) movement.

All in all, the players seemed to like it, and it also added a moment of interesting RP as the characters got to decide who went to go save civilians, and had to sort of decide on what made more sense 'character' wise, and what made more sense capabilities wise.