It basically goes like this: espresso, pizza, gelato, pasta, repeat.
The food in Italy is amazing. I mean really truly madly deeply amazing. But beware: there are many cheap (or not so cheap) impostors! A good rule of thumb is if you spot a restaurant with any of the following qualities, just stay away: menus on large boards with photos of the food, anything in a direct line of sight of any major tourist attraction, or men who are basically trying to grab you off the street and into their restaurant.
Let me introduce this first restaurant by stating that I once went to Rome solely to eat here. Yes, it's that good. You will probably want to visit it more than once, so plan accordingly. La Fraschetta (Via San Francesco a Ripa 134 in Trastevere) looks like a tiny hole in the wall from the outside but is a treasure trove of delicious Italian food on the inside. You must, and I mean MUST, order the bruschetta al pomodoro (see below). It may appear to be simply bread with a tomato on it but do not be fooled. It is heavenly bread drizzled in the most delectable of olive oils, toasted in a wood fire oven with the juiciest slices of seasoned tomatoes on top. Just do it. Don't feel bad if you want to order it in place of desert. It's worth it. My favorite pasta is the Fettucine alla Boscaiola with mushrooms, peas, panchetta, and a tomato cream sauce. The pizza is amazing as well. Tim ordered a simple pizza con prosciutto and I couldn't stop stealing little bites. They have great house wines and the panna cotta would be my choice for desert.
La Scala (Piazza della Scala 60 in Trastevere) serves some nice and simple pastas that are great for lunch. I enjoyed the spaghetti with olive oil and red pepper flakes and find that sometimes the dishes with the simplest ingredients can be the most difficult to make. But if you chose only one place in Trastevere to eat at, make sure it's La Fraschetta.
If you're out and about sight seeing in the city center I would stop by Colline Emiliane (Via degli Avignonesi 22). Established in 1931 this small restaurant is somewhat hidden yet conveniently located near some of the top sights. The pumpkin ravioli would be my pick!
I am also a big fan of visiting the local supermarkets for lunches and snacks. Since I lived in Trastevere (and it's also one of my favorite neighborhoods) I frequented Supermercato Standa on Viale di Trastevere 60. You can enjoy great breads, cheese, salami and wine on a small budget. If you're in the city center near Piazza Navona or the Coliseum I would stop by Despar (via Giustiniani 18b). It's a basic grocery store but can be a life saver in the land of over priced fruit stands and cafes targeted at tourists. While shopping on Via Cola di Rienzo you can pick up some afternoon snacks at Billa (Via Cole Di Rienzo 0171-173 located inside of the Coin store.) If you want a fresh market make sure to visit Campo dei Fiori (which we will visit again in the shopping guide!) or pop into the local cheese or butcher's shops. Just be ready to test out your Italian skills!
For local treats to take home I like to peruse Castroni (via Cola di Rienzo 196). They have great dry pastas, spices, chocolates, oils, balsamic vinegars and limoncellos. These make great gifts for your loved ones!
Make sure you enjoy a lot of cappuccinos, lattes and espressos. Drink them at the counter like a true Roman or you'll end up paying extra for table service. I like to try out all of the different pastries each cafe has to offer. I usually wander down windy side streets until I find a cafe that has some charm and looks interesting. I don't think I ever came across a cafe that wasn't any good. The espresso is so good that they would have to try pretty hard to make it bad.
The same goes for gelato. I usually pick spots based on how fluffy and rich their gelato displays are. There are some famous spots (I'll let the existing guide books tell you about those) but I recommend discovering your own! I came across a fantastic little spot nestled against a cobbled wall full of ivy that I would have never found in a guide book. I stumbled upon it while browsing tiny streets full of antique shops not too far from Piazza Navona. My favorite flavors are pistachio, Nutella, coffee, or lemon on a hot day. If you are near the Trevi Fountain you can stop by Melograno Gelateria on P. Di Trevi 101. Near the Vatican there's a great little spot called Blue Ice (Via Ottaviano n.7) that serves up a good gelato as well as some other sweet treats.
Enjoy and buon appetito!