Much depends on how long the day is after you deduct the time needed to get to and from the docks.
On my first visit I spent the first day getting a feel for the old city by using the route 28 tram service which goes around most of what's worth seeing, using a day pass and jumping on and off when something took my eye. The day pass currently costs ÃƒÂ¢Ã¢â‚¬Å¡Ã‚Â¬3.85 including .50c for the purchase of the reuseable 7 Colinas
card that stores your credit. You can also use this pass on all other Carris bus and tram routes and on the metro so you could use it to get to and from the boat. They do market a tourist tram with guide, a ride on that will set you back ÃƒÂ¢Ã¢â‚¬Å¡Ã‚Â¬17.
West to east, the three main districts of the old city are roughly Bairro Alto (on a hill), Bairro Baixa (sea level commercial centre) and Alfama (on another hill). When you see the hills you will realise why the tram is handy!! Alfama includes the castle which has a superb view over the city centre.
Further out from the old centre is Belem, famous for it's tower (which you will probably pass on the ship anyway), the palace and egg custard tarts. There is also the Oriente district, north of the centre past the airport, which was the Expo 98 site, this is supposed to be a tourist attraction but I'm not convinced.
Lisbon transport company Carris website is at http://www.carris.pt/en/index.php
although like most Portuguese sites there are bits that don't work! These include the link to the Tourist Tram page http://www.carris.pt/en/index.php?area= ... _tur_elect
The tour company may try to sell you trips to Sintra (which is worth seeing) and Cascais (which isn't) but if you've never been before you don't need to leave the city.
PS - by the way, I've always said that crossing a road in Lisbon should be classed as a dangerous sport for insurance purposes so be careful!