I'm in as well. I have two challenges. The first (and most important to me) is to learn how to write three kinds of novels (by successfully writing them): Mystery/Thriller, Science Fiction, and Romance. I don't really know how to do this so I anticipate that it will be a good challenge.
Second, I want to learn a language. Currently my husband and I are considering one of these three: Greek, Latin, Italian, or Spanish. We will settle on the language after January 14.
We would also like to learn to dance, which is something that is very difficult for us and both a physical and a mental challenge. However, we plan to put that one off until we see how the finances and other priorities look for the year.