We parked our car at my sister-in law's house when we flew out of there last August. She parked the car for us to pick up when we returned, and even a few hours cost $$. The parking is limited and very expensive (at least $20 per day). If there is any way to get a ride, it is worth it.
We chose HPN to fly Airtran, which had a good rate at the time. Airtran now flies out of Newburgh (Stewart), and we flew out of there to Tampa over President's week. Parking there is much more reasonable and abundant.
Both airports are very small, and fairly easy to get in and out of.