You've received great advice, so I'll just add two little bits. If you like culture and food, you shouldn't miss having dinner in Alfamadistrict in a small restaurant with a live fado music performance. Avoid the touristy restaurants at all costs and you'll have a unique experience. I haven't met a single person who did not enjoy Portuguese food! (dislcaimer: I am Portuguese....
)
In Bairro Alto (another district) you will find a very eccletic and buzying ambience with different kinds of bars and all kinds of people, if that is your thing.
Just note that in Portugal, life happens late... No dinner before 7, best after 8 or 9. And the bar scene also starts even later, so that might be a problem arranging transportation back.
Anyway, do not miss Alfama for dinner at any cost.
Finally, do not miss:
- Castelo de Sao Jorge by the end of the afternoon for a great view over the city and river
- all Belem monuments - AND not do not miss the traditional custard "Pastel de Belém" in the original pastry shop that invented them
Good luck!