There are many things to see and do in Lisbon. Almost every tour will include a visit to the Mosteiro dos Jeronimos (the Hieronymite monastery), located in the Belém section of the city, along the river. Here you’ll see intricate royal tombs and some breathtaking stonework in the interiors. While you're in the neighborhood, go check out the Torre de Belem, an unmistakable white tower built int he sixteenth century to honor Portugal's naval prowess.
Just outside of the city itself (on the way to Sintra) is the Palace of Quéluz, one of several palaces once owned by the Portuguese Royal family and now open for visitors. One place that is often overlooked (but shouldn’t be) is Lisbon’s tile museum (museu do azulejo). Portugal has been making fine tiles and ceramics for centuries (as you’ll soon learn if you visit) and this informative, interesting museum is dedicated to them. The Lisbon Cathedral is a bit hard to find and really not as interesting as the Hieronymite Monastery, but cathedral buffs may want to check it out. St. George's castle, on a hill in the middle of the city, has been a Lisbon monument for centuries.

If you’re sick of cathedrals and palaces, go check out the Oceanarium, a new attraction featuring the largest aquarium you’ll ever likely to see, numerous smaller aquariums and several reproduced habitats: all populated by marine life, of course. Lisbon also features an aquarium, but it is much older and not to be confused with the Oceanarium. There is also good nightlife in Lisbon and a handful of large shopping malls for those visitors who have had enough history. 
The Baixa neigborhood is a pedestrian zone several streets long and across, and is a great place to shop, eat and people watch. It ends on the water at Constitution plaza.