Reddit mentions of Top 10 Rome (Eyewitness Top 10 Travel Guides)

its a good time to go, not too many lineups for the touristy stuff. I enjoyed the parthenon, walking around the forum, coliseum (watch the scumbag gladiators, they will offer to take your picture then try to extort you for significant amounts of euros) and the vatican of course. Nothing too off the beaten path really. The cappucin monk crypt was kinda funky/spooky. Fontana di trevi, and about a billion churches. Just fun to sit and people watch, and enjoy the cuisine. Never take anything from anyone - nothing is free. Also tell your girlfriend not to take anything when offered, flowers for instance.

This book is awesome to make the best of a short trip:
http://www.amazon.com/Top-Rome-Eyewitness-Travel-Guides/dp/0756669472/ref=pd_sim_b_5

Have fun and be safe!
edit: oh one more thing, its really chilly in winter, gets super damp so dress more warmly than the temperature would suggest.