Cascais, my town!! Really glad you enjoyed your stay!
the rich feel is because cascais and estoril always been the place where the kings come to the beaches, altough cascais was for a long time a poor fishing village, the turism changed it a bit i guess..
Sintra is beautifull, you need to visit the palaces and castles around sintra, but go on the off season of you will be stuck in lines everywhere..
and guincho is for locals, because when there's no wind its from far away the best beach of Cascais!