The Ristorante Nuraghe (formerly called La Strega) is a lovely restaurant in the northern part of the city. It is easy to reach from the city centre by public transport. Take the U1/U2/U3 or U8 until Lindenbaum (about 10 minutes from Hauptwache). The restaurant is just next to the stop.
The interior is friendly and welcoming. The service is good, attentive but not constantly hovering around you. The prices are reasonable.
The restaurant has a good selection of starters, salads, pastas, pizzas, meat dishes and seafood. But they are all a little different compared to your normal Italian restaurant. My favourite pizza is the one with potatoes and thyme. An absolute dream.
Definitely worth a visit if you are up for getting out of the centre and would like to have a nice evening in a cute restaurant.