I'll second the recommendation to leave a pair of shoes at work. Even if you don't want to ride clipless (which would make a world of difference over 17 miles), you might want to have a clear pair of shoes at work, since 17 miles in the summer in CA can make the feet sweaty.
A towel if you're going to take a shower at work - nothing sucks more than using paper towels. Also, shampoo, soap, etc.
Sunscreen, for the ride home.
Anything you might normally keep in your vehicle (like an ID badge?)
Good luck!