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!
Cascais is almost a coastal commuter suburb of Lisbon, it's less than 20 miles centre to centre. There are frequent local trains running back and forth. There is hardly any break in the urban sprawl, the next place inwards is the resort of Estoril which at one time was quite upmarket but has faded a bit over recent years. Cascais and Estoril were the original portuguese package tour resorts when air travel became feasible but lost out because that air travel led to development of bigger resorts on the algarve where hot sunny weather is almost guaranteed during the summer. Cascais is still holding up and is well maintained but doesn't get as many foreign visitors.
It would be quite a good base. As well as being easy to get into Lsibon there are direct local buses to Sintra once or twice each hour avoiding the need to travel via the city. Most of these make a little detour to Cabo da Roca which is the most westerly point in mainland europe and has some spectacular cliff top views. From Sintra on some days (and it's never clear which until the season starts!) there is a vintage tram that heads off through the hills and pines until it reaches a little beach side village called Praia das Maçãs. It's a nice little ride and a nice spot for lunch and the beach.
The only real negative is that the train from Porto uses a different station in Lisbon and you'd need a taxi across with luggage. But on the other hand the Lisbon Airport express bus runs from the Cascais line terminus at Cais do Sodre on the way home!
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).