Hello- Just throwing this out, we picked up our rental just outside Sintra. The Sixt office was about a 10 minute taxi ride from the Royal Palace. If you are heading Lisbon, Sintra, then driving around, this will save you a few days rental and not have the parking issue in Sintra. Plus, you start your drive in a suburban area.

It's a real easy train ride from Lisbon to Sintra.