Invite just a small group of four or five friends over for a very relaxed, very casual after-Christmas afternoon party. Everyone might be Christmased-out, so keep things light and stress-free. Many of your friends might be away for the holidays still, but those who remain or who have returned already may be grateful for a respite from family. It's great to relax with good friends after potentially stressful family visits.
Serve light snacks and appetizers, avoiding a big heavy entree as your guests might be overstuffed from the previous day's festivities. If guests come in the morning, make omelettes and serve festive oatmeal.
Put on football, soccer or a funny non-Christmasy movie for people to enjoy. Set out board games, card games or other diversions.
CHRISTMAS BOXING DAY PARTY DECORATIONS:
Keep your Christmas decorations up, but tidy up for the party. Put away gifts you've received, extraneous wrapping paper and ribbons, and stacks of gifts you might be waiting to drop off at someone else's house.
Invite local friends and family members over (long after their morning gift-opening time) for brunch. Keep it stress-and-obligation free. This is a great party for people who aren't traveling to visit relatives and don't want to be "alone on Christmas" but need downtime too. They can come to your brunch, go home and relax in front of their fireplaces. Serve a hearty festive brunch, encouraging people to eat comfortably on your couches rather than sitting formally at your dining table.
The great thing about this party, is you've likely already decorated for Christmas long ago. So your setup and prep time for this party is already done!