There are several day trips you can take from Lisbon but Cascais & Sintra is a perfect one. Cascais is about a 40min train ride along the coast from Cais Do Sodre train station & only a few euros, you literally can't beat that. In Ireland a bottle of water would cost you more. You arrive into this tiny beach town, which was a favourite of King Luis 1. So what does one do in this little town? Well you can wander in & out of the multitude of tiny shops, eat lunch overlooking the ocean, visit the Citadel, lay on the beach, sunbathe, drink some Portuguese coffee or porto in the tiny cobblestone streets.
After a beautiful morning in Cascais, we got on a bus & headed to Sintra for the afternoon. On arrival, there is so much to do that you could literally spend an entire day in Sintra. Take note that it is high in the mountains so it is a lot cooler than Lisbon or Cascais. We wore t-shirts in Cascais & bought hoodies that evening in Sintra. Sintra has a bus system that will take you to all the palaces, castles and museums for E5. They leave every 20min. Sintra town is gorgeous, filled with restaurants, shops and bars, here you have the first palace and a museum. We decided with time ticking to head to Pena Palace. En route we passed Moorish Castle, it looked amazing & had views of Sintra below. Ten minutes up the road was Pena Palace, E15 entry fee & you are free to walk through the entire palace. It is a UNESCO world heritage site & one of the wonders of Portugal with construction starting in the Middle Ages. You can see all the rooms and walk around the grounds. The views are amazing but the wind would cut through you like a knife. They saw on a clear day you can see it all the way from Lisbon. After head back into the town for some shopping & a bite to eat. The small narrow streets invite you to taste more porto & coffee. You can take another 40min train from Sintra back to Rossio station for pennies. The trains have no toilets, make sure you go before.