If you have a back, then you probably need a Deuter Futura 28.
This thing is the best small backpack you could possibly imagine. I'm on a year long trip around the world right now, and this is my sole piece of luggage.
What makes it great?
First, it's small enough to pass for a regular backpack, so it can be used around town or in the wilderness. It's not one of those huge bulky bags that hardcore campers wear. At the same time, it is built by a hardcore camping company.
The big feature is that it has a lightweight spring steel frame built in. This keeps the bag off your back, so your shirt doesn't get all sweaty and gross. This also distributes the weight properly to the hips, so it's very comfortable to carry around.
It has one large compartment that can be accessed from the top or bottom (VERY smart), or you can zip one zipper and convert it into two compartments.
It has a built in rainfly that zips out of the bottom. I would personally prefer that the whole bag is waterproof, but this is almost as good. That would be my only suggestion to Deuter to make this bag even better.
The bag has all of the other extras you'd expect from a high end backpack: adjustable straps with chest and waist belts, space for a hydration pack, a smaller front pocket with lanyard, and mesh side compartments for a water bottle or other gear.