Cascais has many restaurants with many different types of cuisine. As per usual, the best are a little off the tourist track. And there are a few with petiscos. Prices vary from very reasonable to very expensive IMO. But, they are generally a bit higher than some of the towns between here and Lisbon.
There are not a huge number of sights but there are several small but pretty beaches, a couple of lovely walks along the water (both bay and open ocean), several places to eat and/or drink with water views, a wonderful Mercado twice a week, a fort, a number of historical buildings and great calçada in the downtown area. Also depending on when you are here, the city sponsors festivals and special activities on numerous weekends into the fall.
Depending on how late in the fall you will be here, the downtown area might be a good choice. It gets noisy in the summer but quiets down a lot after the high season. The downtown area is very walkable and there is a bus (Buscas) that makes a loop around town that is helpful if you choose to stay out of the downtown area. There are many, many vacation rentals, as well as, hotels in the area - depending on your price range.
You also might want to look at the Casa da Guia area for a place to stay. It’s on the open ocean, with a great walkway to town and would certainly be a quiet area as it is mostly residential.
