I was there back in 1991 -- Similar story, working in Germany, wanted to flee the cold of winter, so I headed to Spain and Portugal.
My main memories of Lisbon --
*) I found a room in a guest house for just $7 a night. Sure, I had to share the room and the bathroom, but c'mon -- seven bucks!
*) I befriended a stoner who spoke better English than most every German I worked with. His English teacher? American movies. This lead to a personal theory of mine: People in countries that subtitle American movies speak much better English than people in countries that dub American movies. I don't think this has yet been disproven.
*) The people are generally laid back and very friendly. I had no trouble striking up conversations and generally palling around with the locals.
*) The city is extraordinarily beautiful. Bring your camera. It's simply spectacular; one of my favorites in Europe.