The sights in Xi'an are great.. but the food is even better :-) Plenty of good advice upthread on visiting the terracotta warriors (which I just did yesterday!), but I'd seriously consider a food tour on your other day. Over the two days, make sure you try the various local noodle specialties, lots of rou jia mo (which I love), and some yang rou pau mo (more for the experience than the taste in my opinion). And plenty more beyond. Lots of good street food in the Muslim quarter and elsewhere too.

(caveat: I'm no expert, just been lucky enough to make a few trips here for work).