We plan to make a side trip to Sintra .. how long is needed for the essentials? Any city or guided tour?
You can see the highlights in a day and by zipping from Lisbon or two days at a more relaxed pace. There's no need for a guide, just absorb advice in Rick's Portugal guidebook.
A day trip from Lisbon by train gives you time to see the historic sites in Sintra.
Well ts, nothing is really "essential", but I stayed 2 nights and would have enjoyed another. The town of Sintra is lovely and relaxing when not crowded, with many sites, much public art, many parks and green areas, and plenty of interesting shops, bakeries and good restaurants. There are about 5 close-by palaces, each different and enjoyable, most with an estate or park to wander around. Each would merit a half a day or so. In a day tour you can see a couple of these and have lunch, although you will probably be there say, 10 to 4, when the town is most crowded with other day-trippers. I understand the most crowded day is often Monday, since many sites in Lisbon are closed on Monday. I think you can easily visit Sintra by train with a good guide book, although a tour might get you around more easily, with a guide to answer questions, but you pay for it, and lose flexibility by being part of a group. The train station is about a 10-15 minute level walk to the center of town, a nice walk with a lot of statuary and parkland along with way. Enjoy lovely Sintra and wonderful Portugal!
I agree with @Larry - I'm Portuguese and I've been there and I can tell you that you'd need at least two days to get a grasp of the beauty of Sintra (one of the most beautiful places in Portugal).
There are many wonderful places where you can lose yourself like Quinta da Regaleira, Pena National Palace, Sintra National Palace, Monserrate Palace and most of them have beautiful parks to explore... There's no way you can enjoy Sintra in just one day tour... You could also spend the night there so you'd avoid some trips from/to Lisbon ;)
Enjoy Sintra ;)