Take the Ferry.
You will be right next to the Exploratorium and there are plenty of other things to do around there and great restaurants....
I agree. Take the ferry from Larkspur. Take a jacket, for sure, and a camera.
According to google maps:
Larkspur to Exploratorium via 101 freeway 16 miles, 29 minutes
According to http://goldengateferry.org/schedules/Larkspur.php:
Larkspur to Ferry bldg./Exploratorium 30 minutes and $11.49 per adult
I personally would drive, since I know the area. But the ferry is much more peaceful, beautiful and fun.
Have fun. The Exploratorium is really nice. And it is fun the see the skateboarders and cyclists in the area.