For a one-day visit to Lisbon, here is a well-rounded itinerary to see the main attractions and enjoy the city's highlights: Morning:

• Start your day in the Baixa and Chiado neighborhoods. Walk around Praça do Comércio, a grand waterside public plaza with outdoor cafes and shops.
• Visit the Santa Justa Elevator for lovely city views and then stroll to Rossio Square, known for its distinctive cobblestone wave pattern and fountains.
• Consider a quick visit to the Church of Saint Dominic nearby, a somber and historic church with a unique interior.

Afternoon:

• Head to São Jorge Castle, a must-see landmark perched atop a hill with panoramic views of Lisbon. Walking the castle walls is highly recommended for the views.
• After the castle, visit Miradouro de Santa Luzia, a beautiful viewpoint in Alfama with a tiled pergola and views over the Tagus River.
• Optionally, visit Miradouro das Portas do Sol nearby for another striking viewpoint.

Late Afternoon / Evening:

• Explore the historic Alfama district with its medieval streets or visit the Lisbon Cathedral.
• Walk through Baixa and take in the sights on Rua Augusta.
• End the day in Bairro Alto, a lively neighborhood full of restaurants, bars, and nightlife for dinner and drinks.

Other notable sights if time permits:

• Belém Tower and Jerónimos Monastery (UNESCO World Heritage Sites) in the Belém district.
• Time Out Market for a variety of Portuguese food.
• Take a tram ride on the iconic Tram 28 through some historic neighborhoods.

This itinerary balances outdoor viewpoints, iconic landmarks, historic neighborhoods, and local dining for a comprehensive experience of Lisbon in just one day.