No no no no no! Catch the train - much more comfortable. Trust me on that!
One-way is around EUR30, EUR40 in first (very comfortable).
You could even break your journey in Coimbra.
Lisbon Sintra is train, Lisbon Cascais also - but completely different lines, different stations. Both lines (and stations) very very crowded in the morning rush hours into Lisbon, the evening ones out of the city - so you should be OK
Between Sintra and Cascais there are direct buses, and others which call at Cabo da Roca on the way:
http://www.transporlis.sapo.pt/Defau...012&j=0&mobR=0
I'd suggest Sintra first, then Cascais/Estoril so you can enjoy the end of the day by the sea. I love the paved walkway above the beach between Cascais and Estoril (take the train or taxi back to Cascais, or take the train onwards to Lisbon from Estoril). If it's not raining, it's not too late and if you're not worn out from Sintra.....
It will be a long day! You might decide to do Sintra and the Cape one day, Estoril/Cascais another.
You should save time to take the ferry over to Almada (on the other side of the river from Lisbon): it's a short but interesting trip, the view from Almada over to Lisbon makes a terrific backdrop for lunch or coffee. You can go from the ferry dock next to Cais do Sodre rail station (for the Cascais trains).
Lots to do - have fun!