We will be spending 4 days in Rome the beginning of November. I'm thinking of taking one of the days to do a day trip outside of Rome for a change of pace. I'm considering Ostia Antica (+always wanted to see these ruins & quite easy to get to/-more ruins after already a lot of ruins) or Orvieto (+a real change of pace and a chance to see the countryside and another area of Italy/-a bit far and we would miss Ostia Antica) or ??. Your thoughts?
I have to preface this by saying that I don't think you can see too many ruins.
However, Ostia Antica is a nice, peaceful, as close to the country as you can get in Rome and a very different set of ruins than most of the ruins you will see in Rome. Rome's ruins are primarily monumental, big temples, big arches, big marketplaces. You can see domestic ruins on the Palatine, but these were the houses of the rich and powerful, not your average Roman's. Whereas in Ostia Antica, you can see the ruins of an ordinary town, there are shops, roads, houses, city hall, small temples, graves, guilds, synagoge, etc. These are the unmonumental ruins of ordinary folks in a pretty complete layout, you can really get a feel for how a Roman town woud be and that is pretty cool stuff in my opinion.
It is also easy to add a stop (on the same train) at St. Paul's beyond the Walls, w/an intact preserved 4th century basilica plan, helps the basilica ruins in the forum make a little more sense!
I've done Pompeii as a daytrip from Rome. It's a long-ish day, but Pompeii really is fascinating!
I've been to both and you really can't go wrong with Ostia Antica or Orvieto. As you mentioned, Orvieto will be more of a change of pace and the views of the countryside are amazing. However you may need your 4 days in Rome just to see Rome.
We will be in Rome in mid November--our fourth trip there and will visit Ostia for the first time. We went to Orvieto on the last trip and loved it. But we spent the night and I think just doing it as a day trip would not be enough--it is very special at night.