IMHO the Ming Tombs are a waste of time. It really depends on what you have already seen. There are a tons of secondary places to see such as the Marco Polo bridge along with the adjacent museum plus many other interesting places. For example, very few people go to the National Museum even though it is right next to Tian'Anmen.
Also with 2 days (assuming that you can leave Beijing and not on a TWOV), Chengde is a good option.