We were in Lisbon this summer and absolutely loved it!
I assume you will have a car since you are "leisurely" driving to Spain, so may I suggest a must see stop just outside of Lisbon, to the mountain village of Sintra! This pueblo is a world heritage site and has a quirky, beautiful castillo with self guided tours and beautiful views of the coastline and mountains! Well worth the trip and it is only about 30 minutes outside of Lisbon.
Don't forget to stop by the seaside barrio (in Lisbon) of Belem and have a bica (strong coffee) and a pastel de nata (custard pastry) at the famous bakery there.