I've booked two extra nights in Naples since we want to see Herculaneum and the RS tour doesn't include a stop there (it does stop at Pompeii).
It looks like a single day to Herculaneum is enough (and we'll probably also visit Mt. Vesuvius that day), but I'm wondering what to do on the last day. We could just walk around the city and take it easy, but I find that having a specific activity planned is better for us than just a completely free day with no plans.
The tour does include these things:
Walking tour of Naples
Naples' National Archeological Museum
Are there any other activities we should think about on our last full day? It could involve a train trip, we'd be willing to take a train ride to an interesting destination. It will be a Sunday (if that matters).
Thanks for your suggestions.
edit: I didn't mention that the Rick Steves' tour also spends 2 nights in Sorrento and we will previously have had a chance to visit Capri. So those suggestions are great, but already covered.