I visit Billings several time a year because I have family there. The dining scene used to be pretty barren, but it's much better now.
Enzo's, tho still in business, seems to have lost most of it's charm & good food.
The two restaurants that I can personally recommend are Stella's Kitchen & Bakery and Cafe Italia. Stella's for breakfast & Cafe Italia for dinner.
Both absolutely worth a try.