Cyclocross is worth checking out, but might be too aggressive. Another option is the sport/touring class of bikes. They are incredibly versatile, allow for tires up to 40 mm or more, or as narrow as 28 for faster road riding.

For example, check out the Salsa Vaya 3. I was very close to pulling the trigger on this last year but in the end went with the Casserol as I didn't really need something that could handle a lot of dirt and gravel. Happy with my choice, but if I rode more dirt, gravel, or crushed limestone, I would have gone with this.
http://salsacycles.com/index.php/bikes/vaya_3

Though the Casserol is discontinued, QBP sells one under the All City Brand, it is called the Spacehorse. http://allcitycycles.com/bikes/space_horse

Finally, the Jamis Bossanova is worth a look. My wife was looking at this one, but in the end went with a Satellite road bike instead. But, if she really wanted something a little more durable, this bike looks really attractive for the price.
http://www.jamisbikes.com/usa/thebik...nova_spec.html