Unfortunately I don't have much to add for you, as I am working on a similar event - and will be stealing ideas from here to fill in some gaps.

But here's a couple of morsels I've got...

- Two of the guests are having a secret affair/relationship, and arrange a brief rendezvous amidst the activity. This could be a matter of station or situation - noble vs common, species involved, one side is married, etc.
- A lot of deals and power plays. While preening and posing and generally being nobles, alliances and arrangements are being formed. Depending on the role of your PCs, they could be witnesses, or brought into deals, or have plot hooks thrown at them.
- A recounting of the year/time since he last such gathering. Drop some setting info on 'em if they listen, or note this is happening while they scurry off to look at more surreptitious doings.
- An eccentric or exotic choice of doorman. Unusual race, a magician or animated suit of armor, etc. My go-to is a monovocabularic Ogre with a ruff ("Door!"), but that might be too soon for some folks.
- Something to kickstart an adventure - theft of a valuable item or dangerous artifact, but you probably already have your plot threads in line.
- Animated trophy animals.