There are somethings you really have to take seriously, like pizza and coffee: Now if you want a slice of Pizza here is my favorite in Rome: Yeast and Flour, piazzale Clodio, Sestilio Ippoliti will take you for a trip with his wit, knowledge and wisdom. 

If you want to sit down: I would go to Giulietta to decide if it is better Pizza the Roman way or the Napoletan way, two ovens, two philosophies. Piazza dell'Emporio in Testaccio. By Fratelli Salvo

And now the coffee, after the excellent Bar Roscioli, here is another place where you can try the toasted beans prepared in 7 or 8 different ways: Bar Due Fontane, Piazza Perin Del Vaga Roma. A lovely place run by people with a lot of passion.