My wife and I are planning a very short 5 day stay in Rome. In addition to seeing the main sights (Colosseo, Roman Forum, Vatican Museum, etc.), we'd like to take a daytrip to one of the following sights. If we had to choose only one to see, which would you recommend in terms of scenery, history, etc.? Also, how long does it take to get there and back from the Stazione Termini, and how much time should we plan for walking around?

1. Ostia Antica (15km southwest of Rome)
2. Tivoli (Hadrian's villa, east of Rome)
3. Catacombs (Via Appia Antica)

The only one I have been to is Tivoli, and it was beautiful. I've been there twice, both in summer, and it is a gorgeous, breezy break from the city. I highly recommend visiting. The first time I took a bus, the second time we rented a scooter. The bus was about 30 min., the scooter took longer (we got a small scooter which was not optimal for this trip). I'm not sure about trains. I would plan on an hour at each villa for walking around, at least. The gardens at the villa d'Este are magnificen, very pleasant to spend an afternoon and picnic.

I think I'd go with Tivoli too. Hadrian's Villa + Villa D'Este make a fine day out. Plan on a full day if you go independantly.

Buses for Tivoli leave from PONTE MAMMOLO bus station - which you can reach by metro line 'B' from Termini.

It's about a 45 mins bus ride - so plan on an hour from Termini.

I find Villa Adriana fascinating and can personally spend 3 hours++ there, and an hour+ at Villa D'Este - but it depends on your interests.

There are tour buses which do both as a half day tour if you prefer.

Hope this helps ...

Steve

For Ostia and Tivoli: Go to www.twenj.com click-on "Rome" and then "Excursions" on the sidebar. For Tivoli it's the same bus Steve mentions just 1 metro station further and a just a different stop along it's route.
For the Catacombs do a search at the top of the page for Archeobus. It's a hop-on/hop-off bus that will take you to them plus other sites in that area.
My 1st choice would be Ostia Antica and the Catacombs 3rd. Regards, Walter

Have not been to Ostia Antica but would certainly choose Tivoli over Catacombs. Can't help with time from Stazione Termini because I was with a group tour.