Café Giolitti
One of the oldest pastry and ice cream shops in Rome (since 1900). In this 'Antique Roman Cafè' you also can order a snack or a sandwich but they are famous for the ice creams. First buy an 'ice cream ticket' before you order.
Address: via Uffici del Vicario, 40, tel.: 39 6 6991243
http://www.giolitti.it

Gusto
Each part of this restaurant has its own style, purpose and type of food/drinks. There's a 'normal' restaurant, pizzeria, wine bar, osteria, fromagerie, and so on. For lunch you make a selection from the sumptuous Italian buffet, put it on a on a steel plate and accordingly the weight you'll pay. Also nice: the terrace outside.
Address: Piazza Augusto Imperatore 9, tel.: +39 6 3226273
http://www.gusto.it/

Capitoline Café
This café has a roof terrace which offers you an amazing view over the rooftops of Rome. It's situated in/on the Palazzo Cafferelli (part of the Capitoline museums). You can enter from inside the museum or go round the building.
Address: Piazza del Campidoglio, tel.: + 39 6 69190564
http://www.museicapitolini.org