Starting your adventure in Lisbon, head straight to the Belem Tower - a UNESCO protected fortress that dates back to the 16th century. From here, stroll over to the Jeronimos Monastery - another UNESCO World Heritage Site that's one of the most beautiful buildings in Lisbon with intricate architecture. Both monuments offer panoramic views of the Tagus River. Before leaving, grab a pastel de nata, the traditional Portuguese custard tart from the famous bakery, Pastéis de Belém.
Start Day 2 by visiting Rossio Square, a popular meeting place for locals and tourists. From there, walk to one of Lisbon's iconic landmarks, the Santa Justa Lift which offers fantastic views of the entire city. From the top, walk uphill to the ruins of the Carmo Convent and the adjacent archaeological museum. On your way back, walk along the colorful streets of Baixa, which is lined with shops, restaurants, and cafes.
For an adventure-filled day trip, head to Sintra on day 3, which is a 40-minute train ride from Lisbon. Once in Sintra, hike through the lush greenery at Pena Park to get to the highlight of the town - the colorful Pena Palace. Climb up to the palace's terrace to enjoy the 360-degree views of the forest and sea below. Before returning to Lisbon, explore the narrow streets of the historical center and try the delicious queijadas - a pastry made from fresh cheese.
If you have more time in Lisbon, consider taking a day trip to Cascais, a picturesque coastal town. You can also explore the trendy neighborhoods of Alfama and Bairro Alto or try some of Portugal's finest wines with a wine tasting experience.
If you have less time in Lisbon, consider dropping the Belem Tower or Jeronimos Monastery from your itinerary. Also, note that Pena Palace has long waiting queues, so it's recommended to arrive early in the morning.