The Trevi Fountain forms an impressive facade to the Palazzo Poli. The fountain is a short walk from the Colosseum, so you can easily see the sunset from the arena before heading down the street. The fountain is great to visit at night because the entire facade of the building is lit by spotlights. The Trevi Fountain grosses more coins from people casting wishes than any other fountain in the world (or so I was told by one of the people on the cleaning crew).