Well actually, I wish I'd stayed longer! Here's why - with a few pics by me and the friends I was with.
The place has about 7 palaces to visit, with large gardens/estates (we only did 3). As a playground to the Portuguese aristocracy from the 1500s onwards, it's got gothic gardens (a tower carved inside the rock at Quinta da Regaleira), lots of exotic plants (some Sequoia redwoods can be found in the Palacio da Pena ground), stunningly designed rooms (the blazons room with its ceiling painted with the emblems of the Portuguese royal family and many others, at Sintra Palace) and more. The Moorish Castle is from even earlier. I found it hard to edit down my pics for instagram!
Sintra is on a hill, which means that sometimes you have to climb up/down steep paths and roads to get where you want to, but also that the view from there will be pretty great: either looking over to the Atlantic ocean or up at the palace rising above the mountain line.
Also, the good thing about hills is that you won't feel guilty for scoffing all the pastries you can buy in the main town. They have pasteles de belem, of course, but also their own speciality, Queijadas de Sintra, and more. We ate a chocolate pudding cake with cream that was so good we had a second one straight away. Sadly I didn't pick up the name.
We took a day trip to Cabo da Roca, the most westernly point in continental Europe. It's a 45 min local bus ride from town (4.10 euros each way). From there, hike along the cliffs to reach Praia da Ursa, a jaw-dropping beach whose remoteness equals peace and quiet. The path is steep, but it's worth it. We stayed about 3 hours there, enjoying the sun, the kick of the Atlantic, and going up one of the rocks (adrenalineeeee!)
In that strange way of small resorts, it also has attractions one wonders who think they will last: anyone up for a trip to the Bonsai Museum? The local council thinks a Museu das Noticias, about the evolution of journalism and the media, is also a good idea. Hum. Opening date is tbc.
We also took a day trip to the seaside town of Cascais, where we gorged on seafood (bought by the weight at Mariscos restaurant) and ice cream (from Santini, a place that is on the tourist trail but well-deserved: cute waiters and one of the flavours was Strawberry with Basil! Delicious).
Finally, we were also lucky to be in Sintra during one of its festas - local fetes, this one for San Pedro. The neighbourhood was garlanded and the square had rides, churros stands, and food places serving grilled sardines and other local delicacies for cheap.
All in all: Sintra is worth more than your normal day trip!