The Pincio offers some excellent vantage points of the Piazza del Popolo. The only annoying thing: during the holidays, the view is blocked -- usually -- but an extremely large Christmas tree that tends to stay up a bit longer than one might expect. In the late spring and early summer, you can get some amazing photos of Rome's skyline before all the summer haze sets in. It's a bit of a hike from the Piazza del Popolo all the way to the top, but it is worth it!