Just because you're stuck in an office all day does not mean Christmas fun can not extend to your workplace. Depending on the environment at your work, it's definitely possible to mix holiday fun with work.
One obvious choice for some fun at the office during the holidays is to have a party. You could have several, in fact. How about a cookie exchange party? Plan to do this at lunchtime one day, and during that block of time, everyone brings several dozen cookies they have made. You have to set a particular number of cookies everyone brings. Because once everyone has an empty plate, they go around the table picking up cookies that look good to them and place them on their empty paper plate. If everyone bought 3 dozen cookies, say, then everyone gets to take home 3 dozen cookies. This is not a particularly unique idea, but one that brings a bit of fun into the workplace.
Keeping in mind whether or not the public visits your workplace, you may choose to decorate. Why not have a Christmas tree decorating event? Everyone brings 6 ornaments and as a group activity, everyone decorates the tree. This is a good way to build team spirit and decorate your workplace at the same time.
Do not forget to institute a "secret Santa" event at work, where you will secretly buy gifts for someone and have some kind of gift exchange. But what about a "Santa's helper" activity? Someone in the group has to begin this on the sly. Essentially, this first person (the only one in the know about how the whole thing began) puts together a little gift. Ideally, it's a basket with a few gift items in it. They may be decorative items, or baked goods or even bath items. Attach a card saying that "Santa's helper" dropped by and bought these items. Now the person who received the "helper's" gift must put together a little something for someone else and – again on the sly – deliver it to the next person. It lasts until everyone has received a visit from "Santa's helper".
Nothing brings people together like a group activity designed to help others. What if your officemates came up with an activity designed to help people less fortunate at the holidays? You may adopt a local family and everyone in the office purchases items for that family. You may choose to purchase Christmas trees for needy families. If the public visits your office often, you might even begin a "sharing" tree and people can bring items to put under the tree for needy families or children. As a group activity, the office workers can then deliver these items to the needy.
The particularly festive office may want to have someone come in and do a cooking demonstration. If there are enough people interested, you can hire a cook or baker to come into your office on your lunch hour and do a demonstration or class. Say you want to bake but do not know what to bake this year. A baker can come in and demonstrate cookies or other goodies you might not have thought to make. Or someone can come in with ideas and examples for the perfect Christmas meal. These ideas are perfect for the environment where people work many hours and are quite busy but still want to do their regular cooking and baking each year.