It should get you to and from home/work/the coffee shop on a daily basis, so go light. That said, make sure it has room for your laptop, tablet, books, etc. And road-test a few styles; it should feel secure slung over your shoulder.
WHY THIS ONE: With a practically indestructible nylon exterior, pockets galore, and an easy-to-secure Velcro closure, Timbuk2’s D-Lux Laptop Messenger Bondage is as tough as it is intuitively designed. (We love that there’s a side pocket for easy access to keys or wallet.) It’s also just large enough for longer outings and easily expanded to hold a soft shell, camera, and lunch for a trip out to New York’s Long Beach.