Certainly you could go through craigslist to find a great deal on something you like, but if that's not your idea of a good time, by all means just walk into a reputable local bike shop (or even REI). Since you are willing to spend $500-1000, there will be lots of great options from quality brands and the salespeople there will help you choose. Be sure to save that budget for some accessories like a proper lock!
For a flat route you might like a singlespeed, which can be nice because all else being equal they are simpler, cheaper, and lower maintenance. Go get a bike and get riding!