I used to live in Chengdu, it's a nice place but to be honest winter isn't the best time to go.  Like much of central China it's cold, wet, and very miserable.  The pollution is also much worse in the winter.  You almost know that your not going to see the sun for several months.

If you do go I defiantly recommend going to see the pandas, but go early (7 or 8am).  Firstly they are much more active early in the morning, and secondly most of the Chinese tourists haven't got there by then, so it's a much more peaceful and calm experience.  I once went in the afternoon and we had to fight our way through hoards of Chinese tourists just to get a glimpse of a sleeping panda.  In fact I would say go early, or don't bother going at all.

There are a few temples and old streets in the city, but they are no different or better to temples you find all over China.  Fine if you have time to kill.  Some of the best things are outside the city.  Emei shan is great.  It's a few hours on a bus from the city and best done with an overnight stay.  The Leshan giant Buddha is also worth a visit.  There is also Qingcheng shan.  Many of my friends like it there but personally I think its overrated.  Its close to the city so is often still affected by the pollution.  Unlike Emei, Qingcheng shan is easy to do in a day if you don't have much time.