My favorite museum in Rome is Villa Borghese. It has several large Bernini sculptures that are fantastic. I like to go at the very end of the day; allow at least 90 minutes before closing. I've never had trouble getting tickets (they are timed, at least the last time I went.)
I've been to Italy four times. There is much more I want to see and do there. I think i could spend the rest of my life traveling around Italy.
And I tend to walk all over Rome; it's a great city for walking.