We did a "secret santa" gift exchange, and I think we limited the cost to $15 - so a lot of the gifts were jokes but it was really fun.
We also got a huge, beautiful, real Christmas tree (we had a great room with 2 story ceiling so the tree was BIG!) and decorated it. We bought some cheap clear bulb ornaments and some "Christmas color" ribbon to stuff in them. Then we bought paint pens and decorated them with our names and other festive stuff. We did roommates on each bulb, but you could do Big/Lils or other combinations.
A cookie decorating event might be fun, too. You can buy a bunch of the premade sugar cookie dough and cut it out and bake the cookies all before hand, then have a bunch of decorations (frosting, sprinkles, etc) and do that at the party. You could then keep them or give them to other houses.
And don't forget your non-Christmas celebrating sisters! (although all my Jewish friends LOVE to help me decorate my Christmas tree!!!)