The Spanish Steps and Colosseum are tourists traps. Don't bother. You will want to see the Panethon, of course, and the Imperial Forum, Baths of Caracalla, and Capitoline Hill and museum. These are absolute musts. If you're staying more than two days, then by all means go to the Villa Borghese (you'll need reservations), San Pietro in Montorio, Sant'Ivo, Santa Maria Maggiore. I also hear you can score good hash in the campo dei fiori.