My wife and I greatly enjoyed the day trip to Sintra. Pena Castle is awesome, and not to be missed. The drive was not too long, our guide was very knowledgeable, lunch was pleasant, and we also enjoyed about an hour of personal time. We highly recommend it. One note, you must "reconfirm" tours with Viator in Lisbon the day before, and ask by phone where they will pick you up.
We went on the Hop-On Hop Bus bus tour as soon as we arrived, and it was a great way to get the lay of the land. We took both the Red and Blue routes. If you can only do one, I recommend the Red Route, and get off at the Belem Tower for half an hour.