Some notes on Sintra from my trip there yesterday. If you want to at least try and beat the rush, you probably need to be at the Sintra train station no later than 9:00. I got there exactly at 9:15 as RS says and there was already a huge line waiting for the bus. By the time they sold bus tickets and I made it onto the second bus, it was probably a half hour later. All told, I didn't actually get into the gates at the Pena Palace until 10:40, and it opens at 9:45. Be prepared for it to be a sardine fest. Even during the week.
Also, in case it isn't obvious, you have to be a really good walker/hiker to see the castles. From the Pena palace to the Moorish castle the only way to get between them is to walk. It's a LONG walk. I enjoy walking so it wasn't that big of a deal, but just be sure you're aware of what you're in for.
Also, the Pena palace is doing a lot of renovations so quite a few of the rooms were empty with nothing to see. The National Palace was probably my favorite of the three RS mentions. I would highly recommend the Quinta da Regaleira. It's not that far of a walk from the city center, of you can hire a tuk-tuk. I did that to get from the Quinta back to the train station. It was well worth the 5 Euros.
One note about restaurants. I tried the Café da villa for lunch. It really should not be in the RS book. It was awful and the service was VERY slow. It also had bad reviews on Yelp, which should have led me not to go there, but since it was in RS' book, I thought I'd give it the benefit of the doubt.