There were some cool mosaics we saw last summer in Ostia Antica. I can't recommend that place enough. Where Rome is crowded and super busy, Ostia Antica was relatively calm and low-stress. We had plenty of time to wander around and check out all the things.

Most importantly though, since the city was abandoned at roughly the same time, it's still an intact city, so you see how everything was connected, rather than just a ruin here and a temple there. You see it as a complete entity.