Sintra is lovely. Cascais didn't live up to the hype, although I can see the appeal if you're an expat who wants to live among other expats, or if you want to stay at a seaside hotel that's a short train ride into Lisbon. I wouldn't recommend going out of your way to visit though.
A couple places perhaps a little off-the-beaten path we went on our trip that I strongly recommend:
A resort in the Algarve, Vila Vita Parc. Incredibly gorgeous resort on a clifftop with stairs down to private beaches. Great food and a ton of on-site activies, bars, pools, and restaurants. We had 5 days here but I wish we had planned a longer stay. If you're in the Algarve don't miss Rio Formosa Natural Park - a few miles of trails and wetlands along the coast with paths and a ton of nature and species.
"Rustic luxury" they call it, at São Lourenço do Barrocal ranch/resort in the Alentejo region. Stunning. Easy access to Monsaraz and Evora. It's a stop for the Backroads group's cycling tour and the weather in March-April was great.
Six Senses in Douro Valley. Lives up to the hype. Took a day trip to a nearby winery, Quinta do Crasto, and recommend their catered lunch. They're quite generous with the pours on their tasting. Took a small boat down the Douro afterwards by BB cruises. Stunning scenery.
We had a *great* guide in Lisbon area for 3 days. He's pretty much all you'd want in a guide and by that I meant he's realistic and very knowledgeable with a lot of life experience outside of being a guide. Happy to share his details if you dm me.