Walking the hills of the city and enjoying all the pastéis de nata you can eat is a great way to experience Lisbon, but the Tagus is such a prominent part of the local landscape that it's worth spending some time on.
An experienced captain will take you from Belem towards the city center, passing beneath the lengthy span of the 25 de Abril Bridge on the way. Your captain will happily chat with you and point out the sights as you sail past them.