WARNING: This is for more adventurous couples since it's a clothing optional facility and you will be hot tubbin' with strangers in the middle of the night.
Mike and I went to the Esalen Institute a couple of weekends ago. I like being naked, so it was fine for me, but it took a little convincing for Mike to leave his fluorescent pink trunks in the car. Lucky for us, the others in our group of about 12 people were younger and more attractive than we had expected.
Esalen is a full-service spa and retreat center that opens up their hot springs to the public from 1 AM to 3 AM. For $20 per person, you can reserve a spot (they don't take more than 20 reservations per night) as early as six weeks in advance over the phone.
Mike is a road warrior, and we headed back to San Francisco after our hot springs adventure. It's about a three hour drive, so I do suggest getting a place to stay in Big Sur.