Proposing to my girlfriend in Lisbon in the end of a March - thoughts on possible locations?

Hi, at the end of March, I would like to propose to my girlfriend somewhere in Lisbon. I would like to find a nice, private, and romantic place, but I can't decide where. Can you give me some suggestions?
I was thinking of Cabo da Roca, Sintra, or Valverde Sintra Palácio de Seteais.
Thanks to you all!