Just chiming in to say +1 on the advice above. I use an old Trek "Jetta" Mountain bike for tooling around. I put hybrid/almost slicks on it and it's great for hauling groceries in my backpack. If you're going to haul more stuff, +1 on the panniers.
Or, you can go totally cool and get a fixie or single speed. That way you can really get a workout. Go to
www.bikesdirect.com for a new fixie but it'll cost about $300. Maybe you can find used for less on Craigslist but they're pretty popular with the younger set.