2 days in Rome only - what should I see?

[follygirl]

My dh is taking me to Rome for 2 nights at the end of September. Our hotel is near the Spanish steps so I think that's quite central.
I don't want to go too crazy but wanted to see the 'best bits'.

Any suggestions?

the Colosseum, site of the old Roman forum(s), the Vatican, trastevere area.