I've never been one for guided tours. So my favorite part of Lisbon was getting to wander around by myself. It's the perfect city to just walk around and get lost, the alleys are even fun to walk though and lots of shops and cafes.
One shop that I found and loved was Lua de Champagne, an eclectic boutique that made me feel like I'd walked into a fashionably quirky friend's closet filled with unique clothes and jewelry- the perfect spot to buy birthday gifts for Teresa and my sister, and of course, a gift for myself too :) I found it by getting lost one day, the best way to find things when you're in a new city.
So if you find yourself in Lisbon, take some time to wander.