Two of Lisbon's most popular options for day trips are Cascais - a beach resort town - and Sintra - a historic village known for its old castles. Both cities are only about an hour away using Lisbon suburban rail line, Comboios.
I chose to visit the quaint and charming Sintra and its majestically quirky storybook castle. There's an admission fee to enter the castle grounds and the castle's terraces - but I think it's worth it to get an up close look at such a colorful spectacle perched high among the clouds. Of course, you'll have to share the view with lots of other visitors and photo-snappers.