Highlight has to be the trip to Evora to see the "Capela dos Ossos", which is a room filled with over 5,000 bones as decor. I took a train/boat ride there one day to see this, and it was worth it!
Quick Tips:
Learn some Portuguese before going. Although many people do speak English, when you ask "Fala Ingles?", they will say no. Don't even attempt Spanish. I was snubbed more than once for attempting to speak Spanish (it's very similar) when someone wouldn't respond to me in English.
Walk or take taxis everywhere. Walking is free, but taxis are cheap and will save your knees from the neverending cobblestones!
Best Way To Get Around:
You have a wide variety, without hiring a car. Walking is the free way, but again, cobblestones are pretty but become quite hateful after walking on them all day. Trams and buses are good, but very crowded. They are cheap, and a good way to see the city. Taxis are the cheapest in Europe that I've found. A ride to central Lisbon from the airport was six euros. Save your knees and back and take a taxi.