Praia de Paço de Arcos is one of several sandy beaches that line the broad River Tejo as it leaves Lisbon to empty into the Atlantic. Like the other beaches it is backed by the busy N6 "Marginal" road and also within a couple of minutes walk of the nearest station on the Lisbon-Cascais train line. The downside of all this is that the water quality here is quite bad and personally I wouldn't swim here.
The beach itself is actually pretty pleasant. In fact there are two beaches separated by a small harbour; the main Paço de Arcos beach (sometimes referred to as Praia Nova) and the smaller eastern beach known as Praia Velha. The main beach is where you will find all the action with its beach cafe. Behind is a wide promenade which follows the coast around to Praia da Torre a few kilometres away.
Praia Velha is a small patch of sand leading onto a little natural harbour where you will find many little fishing boats bobbing around with the Lisbon skyline and bridge in the distance. There is also a fountain shooting water high into the air just off the end of the breakwater here. Praia Velha isn't really the kind of beach you'd want to throw your towel down but it is a great spot to relax and take in the ever changing view.