Roma in 3 days
The city is easily walkable. If the locals tell you "it is far or will take 30 minutes to walk" that generally translated to 10 or 15 minutes walk. The city is great for walking and the locals are friendly and will provide directions if one gets lost.
Insider Tip:I actually had 4 days in Roma however all the key tourist sites can easily be visited within 2 or 3 days. They are also walkable. Recommend getting the Roma pass, the value is great. Get a tour guide for the Vatican and St. Peters, if possible. The castle at the 10 Angels bridge can be skipped. Look for the set menu at restaurants, they are generally a solid value.