I agree with kyt5009, travellers house was a great hostel. Reception is 24 hours, breakfast is free, and they've got 3 macs for you to use the internet for free. They host little events at night, which my sister and I didn't partake in, we usually just went out for a long dinner and wandered around.
We did the hop on hop off bus one day, which was nice, I think we did the historical route. I can't remember everything that we saw, but we did see the Padrão dos Descobrimentos, which is an absolutely amazing monument, and we saw the Belém Tower, which was very pretty, but annoying since there is only 1 spiral staircase, and you'd start to go up and have people coming down lol.
I would say skip San Jorge Castle, it was very boring and nothing really to see, although the view from the top was pretty.
The Facade of the Igreja Conceiçao Velha (
http://en.wikipedia.org/wiki/File:Conceiçao_Velha_-_Portal_Manuelino.JPG) was so beautiful, although we didn't get the chance to go inside. We found it by just wandering around, but it wasn't far from Travellers House hostel.
You've got to get pictures of this:
http://en.wikipedia.org/wiki/Elevador_de_Santa_Justa at night, it's soooooo beautiful! We didn't take it up, but I think someone else told me there is a resturant on the top...? Not sure lol. We didn't bother tho since it just connects you to the higher streets.
We also went across from the city to see Cristo Rei. You can take the ferry across (like $5) and take a bus to Cristo Rei ($3?). It's free to go see the monument, but costs about $7 to go up to the top. We went to the top, and the view was very pretty. Took about 30-45 mins to take the ferry and then the bus all the way up there.
http://en.wikipedia.org/wiki/Cristo-Rei
I hope you have fun in Lisbon, I enjoyed it very much. We were only there for 3 days, but I'd love to go back, the city was very clean and the food was fairly cheap, although we didn't wander around too much at night, since (especially by the ferry) it seemed pretty deserted.