We went to Sintra last year while staying in Lisbon for a couple of days prior to a cruise, so I can answer some of your questions.
The cruise ships dock in 3 different places in Lisbon, so it would help you to plan your day if you can pinpoint the docking location for your cruise. My Holland ship docked in the Belem area, which is about a 15-minute taxi drive from the train station. The taxis are cheap in Lisbon! We rode for about 15-20 minutes several times, and the fare was always about 6 Euros. This is usually about equal of the cost of 2 metro rides and is much more convenient.
The scenery on the train to Sintra was not very memorable (maybe for the reasons noted below), and if my memory serves me correctly, we did not pass any "beach" areas until near the end of the train journey. I think the train journey took almost an hour one way. The trains ran frequently (every half hour or so). I do have one \warning about the train to Sintra. There was no security on the train, and there were two men who were obviously mentally deranged (and this was not just a put-on show to get $$!!!) that ran through the cars screaming the train during our ride. It was very sad. There were also beggars on the train. My husband is 6'1" and pretty buff, so I always feel safe traveling with him, but I just wanted to mention that to you for your consideration. I think you'll be fine, so just FYI.
If you can navigate through it, I think the site you need to check train schedules is http://www.cp.pt/
. The site has a section to send an e-mail, and I've been very pleasantly surprised by the personal service I've received when writing e-mails for tourist info in Europe. We just don't have that kind of personal service in the U.S., and it's great to receive that from attractions in Europe because it's hard to phone to ask questions when you don't speak the language.
I think your reference to a long steep walk upon arrival in Sintra is for Pena Palace. It is a long, steep, beautiful walk, but if you are short on time, the taxi drivers will be waiting outside the train station in Sintra to take you up to the palace. I think that was about 8-10 Euros and was well worth it to save time (probably an hour's walk). We did enjoy the walk back down.
Sintra is spectacular (we went to Pena Palace and the Sintra National Palace). I think it's very easy to do on your own but the timing might be tight. I don't know for sure and everyone is different, but I think it might be hard to do Sintra as a day trip if you don't have a very long port day and sail away in the evening. If you are there for a day trip and have not seen Lisbon, it would be a tough call because there are several very nice sights in Lisbon. However, if you have your heart set on Sintra, Pena Palace is definitely worth going out of your way because we have traveled fairly extensively in Europe and have never seen anything like it! The views are incredible too, and it's just a really nice taste of Portugal.
We loved Lisbon and hope you will too. Have a great trip!