Everyone's right, can't do Rome justice in too short a visit, best to go with Venice. Do your best to get lost in the side streets and avoid the overpriced tourist spots (that would be most of Venice unfortunately.) Having said that, a ridiculously priced coffee in St Mark's Square is utterly unmissable. You might also want to look into heading to Verona by train for an evening if there's a decent opera on. Or go for a morning or afternoon to Padova, only 20 mins by train - explore the beautiful Cappella degli Scrovegni (Giotto's frescoes) and the gorgeous piazzas (and eat tramezzini and drink spritzes stnading round the tiny bars if you're there around dusk - a truly Italian experience.) Buon viaggio!