The kings of Portugal waved goodbye to the ships they sent off on the voyages of discovery from the Tower of Belem. Set partially in the river itself (it used to be further out, reached by boat, but now one can walk), the tower is a little fortress, including dungeons and the like.
It is fun to stand on the terraces of the tower, and look up the river and out to see and imagine the caravelas setting sail. (A caravela is a peculiarly Portuguese boat, rigged with triangular sails, designed for exploring along the coast, rather than crossing oceans.)
Be sure to also look away from the river, straight up the hill, to see the small chapel where the King and the Captain attended Mass before proceeding to the ship and the tower for the departure.
It is easy to reach Belem to visit the tower, the Monument to the Discoveries, and the Jeronimos Monastery. It is a short and inexpensive cab ride from the riverside in downtown Lisbon, and there are also frequent trams. To return, there is a cab stand directly across the street form the monastery.