Sintra in one day?

Stay in Lisbon - it's huge and you'll not get to see it if you waste half of your day getting to Sintra and back.

Sinatra and Lisbon are both great, too bad you only have one day. You could have a good day trip to Sintra with an early start, but you can't see it all. There are two palaces and a Moorish castle, and that does not count walking around the town itself. The logistics of getting their on your own might slow you down. Certainly the train to Sintra is easy but you would need to get to the train station from the ship. A ship excursion could work alright, but it will be tantalizing to only see what they let you see. You could also have a great day in Lisbon. The trams in Lisbon are pretty efficient. Belem is a highlight, and also the castle on the hill. Riding the trams is interesting and fun. You could do some research on the front end to try and figure out how to do it most efficiently. The bad news is you will miss out on something great no matter what you choose. The good news is that you will have a great day no matter what you choose.

Stay in Lisbon.

We stayed in Sintra for the weekend before our RS tour in mid-September 2016. It's lovely in the early morning and late afternoon and evening after all the day trippers leave. To enjoy the palace you want to be on the first bus up in the morning.

I read this, this morning. https://www.gimmesomeoven.com/life/weekend-lisbon-portugal/ She would not recommend going to Sintra for the day.

I don't think the one-day version of Lisbon is so thorough that it's a shame to go to Sintra if that's what you know you want. Sintra isn't that far, but doing it independently (which is eminently practical for people sleeping in Lisbon) may not be a reasonable plan for a cruise passenger who refuses to pay for the ship-excursion.

When you wrote "short day on a cruise", that sounds like 8AM tie-up, 5PM all aboard. Is that anything close? The distances and the logistics would require you to hire a Sintra taxi for the day, perhaps getting up towards the price of two excursions. But if you're on the excursion, they guarantee that you won't miss the boat. Do you feel lucky?

I will add for Lisbon that the Museu Nacional de Arte Antiga, is in its own way, just as good as the Gulbenkian museum. But not everyone likes paintings on the wall. Arte Antiga has had some renovation closures in recent years, so check if you care about that. I walked from there to the Time Out Food Court building, which was extremely good, if touristy.

Edit: When I wrote "Sintra taxi", I meant that you need a private car at your beck and call to "see Sintra" (whatever that means!) in just a HALF-DAY. I didn't mean that you need a private guide. If you have a guidebook, you don't need a private guide. And many taxi drivers don't have quite the English facility needed for good guiding, let alone guide-certification. Taking the (physically tiny) public bus in Sintra between attractions is impossible because of the time involved. The more walking you can do on your own, the better. Wear light colored clothing because there are many no-sidewalk sections of the road.

Thanks everyone for the suggestions, and Hi Tim from Wyckoff - I grew up in Franklin Lakes! I absolutely don't mind paying for a private guide but looking at the itinerary we sadly have only 8-4 in Lisbon so probably have to be back on the ship by 3:30. Such a shame we are not staying overnight, but otherwise it is a really good cruise on Regent Seven Seas. We have an overnight in Bordeaux, but I would have preferred Lisbon for that. Oh well. We will have fun regardless! I really only want to see the outside of Pena Palace - I did read the review of the person who said the lines were ridiculously long to go through it, so we definitely will not do that. I am not much into museums - prefer architecture like castles and palaces or just historic buildings, and just checking out the scenery. Lisbon's trolleys look cute but not sure how much fun they are to ride, or how practical it is to get around on them. Any other ideas for must-see things in Lisbon, and would it be worth it to hire a guide to take us to see just the outside of Pena Palace, our can't you get close enough to see it unless you go inside?

I will echo the other posters - stay in Lisbon. If you want to hire a guide, do it for Lisbon instead of Sintra. A walking tour of Lisbon will give you a good taste of the city without the time expenditure of getting to Sintra.

Especially if the weather is good, Lisbon is lovely IMO. The views from the miradouros are great. If you have time, a trip to the Time Out Market is a fun lunch - if you get there before 12:30. It’s too crowded after that until around 3:30.
And, you can’t really see Pena palace until you get close to it. It requires tickets to the palace then a short ride in a minibus - then walking uphill about 5-10 minutes, before you can really see the palace.

KathrynJ - thanks so much - that really helps to know that you can't see the palace without going all the way up to it. That would take too much time, plus I am a slow walker due to severe arthritis in my feet, so it would take me even longer :). We will stay in Lisbon and I appreciate the food suggestion too!