Carcavelos is the first wide, sandy beach on the Lisbon - Cascais coast. It takes just 20 minutes to get here from Lisbon by car (traffic permitting) along the Marginal road. There is also a train station around 10 minutes walk away, so as you can imagine this can be a pretty popular choice. Whilst the beach is fairly wide, it can barely soak up the throngs of beach-goers who flock here in the summer.
Carcavelos beach is a great spot, and I've spent a fair bit of time here myself. There's always something going on, whether it be beach rugby or a surf competition. On the downside, given its proximity to Lisbon, the water quality can be a little dubious, particularly after heavy rain. I've seen a few things here I wish I hadn't!
The surf at Carcavelos beach is world class. It is one of the most consistent spots in Portugal during the winter and the waves are fast, powerful and often hollow. On a strong southerly swell the waves here can be comparable to the more famous Subertubes (Supertubos) up the coast.