Consider This: Herschel Supply Co. Oak Briefcase, $100

You're older now, which means you need to give the messenger bags and backpacks a rest. They're fine when you're keeping it casual, but business time calls for a business bag. No, you don't need to spend a grip on a crocodile leather one or anything, a canvas briefcase is professional enough to bring to an interview while light enough to carry around all day—and you can always strap it on if you don't feel like holding it.