As a current resident, I think Tucson's lovely year round. I'll echo the recommendation of the Desert Museum--it's really the crown jewel of tourist attractions around here. Make sure to catch the Raptor Free Flight show. Sabino Canyon is also a local treasure, but there are plenty of good hiking options in winter.
Avoid travel in early February, as everything is booked for the gem show (and everything's more expensive).
If you plan on going to Kartchner, reserve your tours in advance.
Cafe Poca Cosa is overrated and overpriced. Much better is the Little Cafe Poca Cosa (open for breakfast and lunch Monday through Friday, cash only). Their food is sublime (and healthy). You might also enjoy the food truck Planet of the Crepes (try the fig, manchego, and almond crepe, and spring for the smoked duck breast addition. The Banh Mi crepe also isn't too shabby. Check planetofthecrepes.com
for her schedule) . If you want authentic Sonoran food, try Taqueria Pico de Gallo (on S 6th Ave at 36th Street), which is usually pretty wonderful. They make their own corn tortillas, so get those instead of wheat. You also might try May's Counter (near The Loft!)--it's a chicken and waffles place, but where the food used to be delicious, now it's sometimes just good. There are also a couple of good Ethiopian restaurants (Cafe Desta and Zemam's).