My partner, his adult daughter, our friend and I will be travelling to Portugal for 10 days in July. We are thinking we will go to Porto and then take a train to Lisbon and spend about half the time in each place. As of now we are not planning on renting a car. We are trying to decide if we will stay in Lisbon or Cascais. We prefer smaller towns and cities and would like to be closer to the beach. That's why we are thinking Cascais. Then we would take a day trip in to Lisbon, Sintra, etc. I'm wondering if there is enough to do in Cascais, if the city is nice and if day trips from there are feasible. Thanks!
If you prefer smaller towns and want to be closer to the beach, Cascais might be a good fit for you. It has a very different vibe from Lisbon and can be quite busy in July - but likely not as busy as Lisbon. It is very easy to spend time in Cascais without a car - I live here without one. And, getting to Lisbon is quick, easy and inexpensive.
Another poster said you would need a taxi across town in Lisbon to catch the train to Porto. That's not exactly the case. You can take the metro from Cais do Sodre to Oriente station. But, it does require a change at Alameda metro station. I've done it with luggage, but if you have a lot of luggage, a taxi would be easier. Plan on about 10€ or less for the taxi.
If you decide to stay in Cascais, feel free to PM me with questions.
I stayed in Cascais for 4 nights at the start of my Portugal trip last spring for 2 reasons: day trips and it's not a city. Sintra and Lisbon are easy day trips. Secondly, which doesn't apply to your trip, is I like the low-key quiet of beach towns in the off season. Not much to do in Cascais except the beaches. It's more spread out than a typical tourist district, so daily I did multiple walks of 600 meters to 1 km. I advise renting a bike, something I chose not to do so because my brain knows to ride / drive on the left which would be dangerous in a country where they ride / drive on the right. Uber service is available.
I recommend a small mom & pop restaurant serving local food to locals. Inexpensive and good. Restaurante Avenida. Avenida Emidio Navarro 23A, Cascais. It's 50 meters further along the road to the SW of Igreja de Nossa Senhora dos Navegantes (Our Lady of Navigators Church).