Lisboa is just great!!! I went there with my girlfriend, and we totally loved it. Lots of streets, going up and down the different hills, nice places where you can have a drink, or eat grilled fishes for less than 8€...
Well, so many stuffs that my 500 words won't be enough, but in a nutshell, for the skepticists (and the other), just go to Lisboa!!

And don't hesitate too long on Barbara's appartment or you will regret it. It is well located in Alfama, which is probably one of the best part of Lisboa, if you want to discover a less touristic part of the city. And the flat itself is nice, cosy and perfect for a couple. No doubt on it : I recommend Barbara's apartment if you want to stay a few days in Lisboa.