Driving from Lisbon to Sintra is easy and allows you to explore the are at your own pace. We chose Auto Europe http://www.autoeurope.pt/ and got the car for a week, since we intended to travel south as well. Took us about 30 minutes to drive from the city center in Lisbon to the highway of Costa do Estoril and following the A5. From there you just need to follow the signs. Sintra tends to get extremely crowded during the weekends, making it difficult to find parking spaces.
If you drive to Sintra and into the old part of the town remember that the roads are very small and there are many one way streets. If you miss a parking spot you will need to exit the town and start from the begining. You are better just parking on the outskirts of town in some of the larger parking lots and walking into town.