Went to Sintra last Saturday, staying overnight to visit Peña Palace Sunday morning. A few tips:
We purchased our tickets online before our trip and printed them out. This avoided issues with our crippled phones, broken ticket machines. There were many unhappy people turned away due to hours long waits at the one functioning machine at the site.
We also purchased the Rossio station to Sintra station a day in advance because some of those machines were also broken. The trains were running both ways, but it is a good idea to know the bus schedule just in case.
Once in Sintra, take a taxi or putt-putt up to the site. We heard the local bus was over-crowded and often late, so sometimes didn’t even stop to pick people up. Besides, you want to save your time and energy for the site itself. Afterwards we hiked down to town on the Via Sesetti. Beautiful, but with lots of very steep stone steps near the top. Don’t even think about hiking up to the site this way unless you are very fit.
The palace and park are amazing. We could have spent another day just walking more of the gardens and seeing the other castles.