Do not buy ice-cream from street vendors, they are well known for extortionate prices, often not until AFTER you have been served and started consuming. I'm talking of 40 euros each is not unheard of!!
The Parthenon is another +1 from me as are the ancient senate ruins around the Colosseum, which is OK for a quick look around. These, the Spanish Steps (for pix) and Trevi Fountain are all within reasonable walking distance of each other.