When there be sure to visit the Alfama area. Also the St Geronimos monastery which has the tomb of Vasco Da Gama in it's church.
The monument to the explorations is well worth walking around & also see the big map of the world on the ground which shows all the areas travellled to from Lisbon.
It's a beautiful city. I went there almost by accident & it took me by surprise. If you get time to take a day trip the upmarket seaside town of Caiscais is also beautiful as well as the old world village of Sintra - not sure if March is too early for that though !
Enjoy