Free afternoon in Rome

If you have not been to San Clemente, DO IT!!!!!!!!!!!!! It was my favorite thing on my trip last month. Going down into the excavations costs about 10E but it is beyond worth it.

Recent reports indicate that the Vatican Museums tend to be mobbed if you don't opt for one of the tours that provide pre-opening access. I'd do something different since you've been to the museums before.

Welllll - we quite enjoyed Ostia Antica! Another suggestion if you haven't done it - head out to Appia Antica - the ancient Roman Road - there are some great crypts there, and it's just cool to walk that road and think of the history it has seen!

The Basilica of San Clemente is very close to San Giovanni in Laterano, if you haven't been there before.

Our pick would be Ostia Antica. Wonderful ruins and history and as different from Rome itself as day and night. Very few tourist, easy to get to using public transit, and will easily fill up an afternoon.

Time of year? Ostia Antica can be a very nice break from the city in the summertime. Very shaded, cool, and generally a relaxing area. This time of year it would be less attractive.

The time of year would be next month (right after Christmas).

You could choose depending on the weather?

It it's sunny and reasonably temperate, go to Ostia Antica.
If it's cold/wet/otherwise inclement, do San Clemente.

I'd skip the Vatican if you've done it before.

Oops: editing as I see Zoe already suggested this but if you haven't been to San Giovanni in Laterano, I'd definitely do that with S. Clemente if you decide against Ostia Antica.

One more vote for combo San Clemente and San Giovanni in Laterano, about 5 minutes walk between them. Look up both of them on Sacred Destinations website, excellent descriptions and photos. Tremendous history and beauty at both, 2 of the oldest you'll find and very different. San Clemente small and intimate, SG in Laterano massive and impressive. San Giovanni was 1st Christian Church in Rome, built in 300s not long after Constantine legalized Christianity. San Clemente much much older if you consider the pre-Christian scavi which are fascinating. I think Rome's December weather, though warmer than Indiana for sure, will put a dent in your enjoyment of either Ostia or Via Appia Antica. These require several hours of walking, with no available shelter. Agree with others to skip Vatican, it's a 4 hour deal even if you don't see everything, not counting waiting in line.

An extra suggestion if you do decide to go for San Clemente: you could complete your afternoon with a visit to the Santo Stefano Rotondo and San Giovanni e Paolo Basilicas - they're both on the Caelian Hill overlooking the Colosseum and no more than 15 minutes away from San Clemente. If there's still time left, I would top those off with a visit to Santissimi Quattro Coronati Basilica, on the border between the Caelian Hill and San Giovanni district. This is just around the corner from San Clemente, also closer to San Giovanni in Laterano if you're interested. My favourite churches in the center of the city, very different from what tourists usually see when they visit, and not at all overrun by visitors.