Oh, lucky you! Lisbon is just a lovely, lovely place.
If you only have one day, I'd say just wander Lisbon. The Castelo Sao Jorge is nice, you get a view of the city and there are great little restaurants and cafes down along the hillside. There's so much to see (not to mention eat & drink...)
If you have another day, I recommend Sintra & Cascais, with the understanding that Sintra is on a very steep hill. Cascais is calm and pretty, too. Less walking, but maybe after Sintra that'll be fine!