If you are visiting Sintra or have friends on their way, do everyone a real favour and ask them to leave their car at home. Parking is expensive and sparse and the old streets were not designed with vehicles other than horse carriages in mind.
If you are visiting from Lisbon for the day, consider taking a train to Sintra and going on foot instead of driving. If walking is not your favorite thing, you can always take the 434 bus to visit the beautiful Pena Palace and Moorish castle. Be warned that in summer time the 434 bus queue can be such that some visitors have to wait for the next bus.
Without any doubt though, the cleanest and most enjoyable way to cover the distance between the parks and palaces is by bicycle - with an electric assisted E-bike, you get the all the fun and freedom of cycling without the effort of pedal pushing up the hills of Sintra, and you will have no trouble at all to find parking. Enjoy!