The first night I was in Rome the first thing I did was walk straight to see the Colosseum at night.  I was lucky I did that because the crowds the next day were massive.
Inside of the Colosseum
© 2005 Matthew Glaeser
View across the Foro Romano with the Colosseum in the far background
The Pantheon
The Castle Sanangelo along the river in Rome by the entrance to the Vatican.
The Fountain Di Spagna which was so full of tourists that day that it took almost an hour to even see it because of the crowds.