This cathedral is spectacular. And what makes it so, is that it wasn't rebuilt after the 1755 earthquake. One of the rare buildings left as is, and its a refreshing change from the rest of Lisbon. There is history here and athough it is marked as an archaelogical museum, don't be fooled. I almost didn't go in because I thought it was just an archaelogical museum. While they have things on display from all over the world, the best part is just sitting on the main entrance steps and gawking at the buttresses that still stand overhead. The sky was such an amazing blue when we were there our pictures almost look fake.
This is definitely a must, and it's the first thing you come to when you get off the Santa Justa Elevator. Just go in! I have no idea how so many things in Lisbon top this on the sights list. It really should be top 5.