A great place to spend a day or more.  I loved the Old Nauvoo area where many of the origional buildings still stand.  Don't forget to get a brick at the brickmakers shop (they are free) or sample the bread at the bakery.  We went a few days before christmas and there were few people there.  The ground was covered in snow and the place was decorated as it was in the 1840s.  Don't forget to visit the Joseph Smith Jr Mansion or many of the other homes.  And the visitor center across from the temple is wonderful - and a great place to warm up in the winter.