This website might help a little bit:
http://www.eslholidaylessons.com/ The lessons are mostly based on small articles and then grammar and vocab review, but at least it could give you some topic ideas for celebrations (=culture) to discuss. You could use it as a starting point to build other activities around.
I also recommend another user's suggestion of going to a bookstore and checking out what they have. Not necessarily to purchase something for your class, but just to browse and flip through books, looking for topic / activity ideas.
Maybe you could turn your class into mini project based lessons, revolving around the major English speaking countries of the world. Break it down into looking at their food/celebrations/music and movies, etc.
And, as others have already said, waygook.org is probably one of the best places to get started, again if only for ideas. As the person looking after this board, I can say that it definitely is not up to date at the moment, but the master list of high school topics would be a good thread to check out. There are tons of cultural lessons here that could be useful to you.