Hey Spike, Great questions. It is not an official holiday on the Thai national calendar, so Thai businesses are open as usual. However, most Christians in the country who are working are usually given the day off if they request it. Businesses owned by Christians will close by their own choice.
Christian churches and schools go all out, and international schools do the same. Almost every Thai, no matter the religion, decorates their businesses several weeks ahead, and some play nothing but Christmas music while shopping. This includes large stores and malls. How individuals celebrate at home depends on the individual, but many do decorate and play music for several weeks before and after the day. Many do all the things you suggested and more.
Here is what I and my friends will be doing this Christmas holiday. Tonight, we are going to Orion Bar for a Christmas party that also has a gift exchange. The bar will provide entertainment, Christmas Music and snack foods. Tomorrow, we will go to a Thai barbeque restaurant for dinner and then to a friend's home for 'afters'. My friend always puts up two trees, a door decoration and outdoor lights. Christmas Day, Aod and I will have a lunch date at a restaurant.
You didn't ask, but the Thais do celebrate the Western New Year officially and to the hilt. That includes all the things you will see anywhere in the world.