Tianjin: A port city that isn't too touristy, but still might make for an interesting day trip while in Beijing as the train is (fairly) cheap at 59 RMB.
Guangzhou: Guangzhou is really close to Hong Kong and has trains that go there several times daily. Not a ton to see here (compared to other large cities, that is!), but it could still be an interesting stop.
Xian: A sleeper train ride away from Beijing, this is definitely a place to visit to see Northwestern China. The terracotta army is close by, along with several other cool places to see in the city (the Bell Tower being one I definitely want to see :). The city wall would be fun to walk, as well.
Shanghai: Shanghai is another port city and is the center of commerce in China. The only downside to visiting here is that I would be visiting in June which is the rainiest month of the year there.
More to come!