We have just returned from a two week stay in Estoril. We were not there for a vacaton per se so our good dining experiences were limited but we have a few gems.
Estoril: Garret's is a bakery/restaurant that makes everything from scratch. Wow what a nice experience and nice people. It is open from 8:00 to 7:00 and gets mostly a breakfast/lunch crowd. Do not miss the pastries. They have a variety of coffees and hot chocolates to choose from. The omelettes and salads were great too.
Jackpot Restaurant and snack bar. They are located very near the casino. At first glance it looks like a bar with counter seating. That is the snack bar side. The other side is a very nice restaurant serving great seafood, steak and traditional Portugese fare. When you are seated they, like most restaurants, deliver a variety of apps. If you accept them you pay for them. We liked all of the apps except the boiled shrimp. Everything we ordered was terrific. Get the prawns cooked on the grill or the steak with garlic or peppercorn sauce.
La Villa - while most restaurants in Estoril were traditional Portugese fare this specialized in contemporary fusion cuisine. It has a lovely view overlooking the water and a very nice atmosphere inside. The food was excellent.
Four Seasons at the Palacio. This is comparable to a starred dining experience in a major city. You do pay for the experience tho. Service was outstanding, food was very good but not great. All in all I would prefer a 2nd dinner at Jackpot.
Sintra - we had one wonderful lunch a Tulas bar which is right down the street from the visitor's center. Traditional Portugese cuisine, decent atmosphere although a bit crowded and a very nice host.