Cachilas is worth a visit for the ferry ride across the estuary alone. But take the time to have a look around (it’s not too pretty, but very different from Lisbon) and sample some of the excellent seafood down at the port while you are there.
Ferries: Every 20-40 minutes from Cais do Sodre.
A pedant writes ... Cacilhas (Caseelyash). Agree with post.