As Carol said, Sintra is lovely but you have to sleep there to appreciate it. Even with a private guide, you'll have crowds.
It's easy enough to start with 2 or even 3 nights in Sintra. Here's what I wish someone had told me before I went: it's a 30-40 minute taxi or Uber ride from the airport to your hotel in Sintra and it's not expensive, even for a solo traveler like me. Outside rush hour, Uber was not much more than €30. Taxis aren't much more. I took the metro to Oriente Station, had to wait in line to buy a ticket, then spent a lot of time going from ticket booth to ticket booth in Oriente (after I found the ticket booths) until I got to the correct one. Then I had nearly an hour's wait for the slow, uncomfortable train to Sintra. Then a taxi from the station.