Cabedelo do Douro is a nice beach at the end of the Douro river. It is protected by a pier which makes it a good flat water spot with the Northerly winds.
Cova do Vapor is where the Tejo river meets the Atlantic. You will enjoy a spectacular view on the Farol do Bugio especially during sunsets. If you prefer more of a beach like feel and good restaurants then prefer the Praia de São João side.
Praia dos Moinhos, in Alcochete, is located across the Tagus river opposite of Lisbon. It's fluvial beach is loved by locals as it is nicely located for all northerlies as well as really good with west winds. West winds are usually much stronger than forecasted as there is a venturi effect with the river.
The Troia peninsula, is blessed with a beautiful beach, sand banks and blue water. It is absolutely magnificent.