I can only comment on Lisbon since Lisbon and Sintra are the only 2 places we have visited so far in Portugal. I suggest spending 3 or 4 nights in Lisbon. It's a wonderful capital city with lots to see and do, and it's wonderful for strolling around and exploring neighborhoods. We loved the Gulbenkian Museum; loved exploring Alfama; Castelo De S. Jorge and the Mosteiro De Sao Vicente De Fora. This monastery was quiet and cool and filled with gorgeous blue and white tiles. Be sure to follow the signs for the towers and rooftop terrace for awesome views of Lisbon and the river.
We spent almost a day in Belem visiting the Mosteiros dos Jeronimos, the Coach Museum, and the Discoveries Monument where we took the elevator to the top for stunning views of the monastery, the river, and the Belem Tower. There were no lines here! You can also visit the Belem Tower but there are usually long lines.
Lisbon also has miradouros, or viewpoints, where you can sit with a drink and enjoy the scenery.
Are you interested in Sintra? Sintra has many castles and palaces. We visited for a day from Lisbon but many people like to spend 1 or 2 nights in Sintra. Cascais and Estoril are nice, too, located on the water. Some people visit as a daytrip from Lisbon, and some people like to stay overnight.
Hope this helps. I am sure others on the forum can help you with Porto, Tomar and beaches.