Several copies of the trail map to keep me inside the ski area.
Making a wrong turn and descending into a "dead end" drainage is the usual reason for lost skiers at ski resorts.
Only leave the trails if you know where you are going.

That said for a day of resort skiing I would have;
SAK Farmer
Mini Bic
Quick tinder in my wallet
Photo freedom on keys.
That is about it.

Out of bounds and BC skiing is a full on survival scenario so I would have a bunch more gear.
Bill