Some people here are recommending a backpack. I disagree. The best thing I ever did was switch from a backpack to a saddlebag. I get much less hot and sweaty on my ride in. I see that you're in Hawaii? Yeah, a backpack will be sweat-central.

I would recommend panniers for your situation (note that a saddlebag is a different thing). I myself use a Carradice saddlebag, which might be a good option also.