Sintra is considered one of the most beautiful places to be visited in Portugal, and so close to Lisbon and Cascais (around 20/30 minutes).
There are several options to visit Sintra from Cascais, depending on the time you have available and your preferences. During high season (March to October), is strongly recommended to hire a private guide, if possible with transportation, to avoid the crowds and the traffic in Sintra, which often can be a nightmare. Public transportation is normally packed and during peak season very difficult to get. If you have a car, you will find hard to park in Sintra, while locals know where to park, foreigners will find it hard, as it’s a small medieval village, the most visited in the whole country.
Sintra is located on the hill, and not having transportation, walking will be required during the day, to reach the castle and the palaces, which are on the top.
Here are some suggestions to visit Sintra from Cascais:
1) Get a Taxi from Cascais
2) Get public transportation (bus)
3) Hire a Private Guide with transportation on a private tour: https://lisbonprivatetours.com/sintra-day-trip-from-cascais/