Torker Interurban or a Jamis Venture Sport. It is important to go to go to a knowledgeable bike shop and try a lot of bikes.
If you want a used bike they will ask you how much you want to spend. At this point don't be as cheap as me when I said "oh, around $100" because in the end I got an old, heavy road bike with a lot of problems. Also old steel wheels have the tendency to go out of true.
For ?650 you could find a used bike that is as good or better than many new bikes, depending on the bike of course. For that much my bike shop would sell me a Tomaso road bike with Campagnolo components and SPD pedals already installed, that he says has been ridden across the U.S.
You could find a classic bike with a Chromoly frame and even STI shifters and good components.
In some cases with a used bike they will even let you take it home for a while.
Even if you are not knowledgeable you could snap a few photos with the phone, identify the manufacturer and level of components, read up and ask questions.
But still you are better off going into a bike shop buying a new model for that much because at least you know it will be reliable and serviceable.
