The coastal area of Alentejo is, on my opinion, the most beautiful in Portugal. To the south of Sines you have a national park called "Costa Vicentina" where you will find beautiful beaches like Vila Nova de Milfontes, and next to Zambujeira do Mar, Odeceixe, Amoreira, etc. You could stay at Aljezur which is a nice place to know.

Not having a car could be a problem because the bus network is not so good in this area.

For the north of Lisbon you have several beaches like Ericeira, Santa Cruz, Nazaré, Peniche with several beaches and the possibility to go to Berlenga, island (some 20/30 minutes) which I advise you to go to. Or even north to Aveiro which is a place I recommend you (some 250 km north of Lisbon).

Also, we could have a GTG in Lisbon. My daugther has 15 years old. If I could be of further support please advise.

Best regards, João in Lisbon