Just be warned that a few years ago, Lady Liberty had some snowman inflatables trashed by the witlesses--they could not take judicial action for having the snowman inflatable.
Should you wish to keep Christmas without being found out about, and your family does not pose a risk of ratting you out, you might put winter themed decorations outside. Clear icicle lights on the roofline (LEDs are more energy efficient), a snowman, snowflakes, even a reindeer or two (no Santa, though) on the outside will tie in this theme. If you dare, a LED candle in the window will also suggest winter--leave these items up until late February or early March to make them look like they are winter items. On the inside, you can go ballistic and do the whole nine yards. You can even listen to Christmas carols at home, have a small gathering, even exchange gifts. If you really want to have a religious Christmas, even going to church for Christmas Eve service is an option.
However, if you are going to be ratted out from insiders, you are more limited on what you can do inside. The above mentioned decorations outside are still good (be prepared to run them until the snow is gone, to avoid suspicion). However, you will not be able to put a full Christmas theme inside--and, for some witlesses, even running red and green tinsel garlands and red lights is too much. Obviously, with a witless on the inside, you will not be able to set up the tree, exchange gifts in the open, go to church (unless you can sneak out during the witlesses' boasting session or Family Waste the Evening night), or play Christmas carols openly. You can still send Christmas cards if you can sneak them out, exchange mail gifts (gift cards make great sneak gifts because they are easy to hide, and can be redeemed in February), and listen to Christmas carols on your MP3 player.
Just watch for any witlesses that have a car full of eggs and are willing to trash whatever you put outside.