Another travel thread - Portugal for a long weekend?

[teriwtt]

So D2 will be in London for the fall semester and we're tenatively planning a long weekend away when I come to visit. I've visited D1 in London when she was there two years ago, so I'm looking to use a few days to go somewhere with D2, I/we haven't been. She will want to do some of her trips with her friends, so it narrows down our list. With these limitations, we've kind of come up with the possibility of Portugal, and it sounds intriguing, but I know nothing about it. Ryanair flies into Porto, but not Lisbon.

If you had three nights to spend in Portugal, what would you do? This will probably be late October.