We have taken several tours by the wehatetourismtours.com people. The one to Sintra/Cascais/Estoril starts in Lisbon at Rossio Square and the guide will tell you all sorts of cultural info on the ride to Sintra. There are usually 6-8 people on the tour. Since you're staying in Cascais, you could just end the tour there, if you wish. Plus, I feel that the taxis in Portugal are quite reasonable, so that makes it easy to get around when you'd like to save some time. Have fun.