Music & Mood Lighting.  Every party needs just two things to be great, both of which are free: music and good lighting.  To set a classy tone, try one of Pandora's old school big band jazz stations like Guy Lombardo, Glenn Miller, Artie Shaw or Benny Goodman.  For lighting, eliminate overhead lights which are harsh and unflattering and place lamps from the rest of the house strategically around the rooms where the party will congregate.

Noisemakers.  Hit the Dollar Store for noisemakers, horns and crackers.  These are not high-quality, big ticket items so you don't have to spend a lot.

Serve champagne alternatives.  Stock up on a few inexpensive bottles of cava, prosecco, or blanc de blancs  and ask friends to bring something bubbly as well.  Also stock up on generic-brand fruit-flavored sparkling waters for non-imbibers.  You can class them up with these savvy ideas. 

Quick, Slow-Cooker Sliders.  Make inexpensive BBQ sliders out of pork butt (very cheap cut of meat) and put guests in charge of nibbles.  If you've provided the main course, you won't look like a cheapskate and the sliders are a major crowd pleaser.  You can also put out a slew of 5-minute tapas if you want to stick with finger foods.

Silverware.  It's fine to stock up on marked down Christmassy paper plates, napkins and cups for the big night, but there's no harm in using your own silverware for easy cleanup and extra savings. 

Decor.  If your Christmas decorations are still out, repurpose lights and hang them around the room.  Balloons are also cheap and fun to leave around for guests to blow up.

Games.  Waiting out the midnight hour can get a bit long-winded if you don't have a plan. Board games are a silly way to while away the hours, particularly if little Dollar Store prizes are up for grabs.  If guests are having a great time mingling, even drawing straws once an hour for prizes can be fun and memorable.