The first question, of course, is what size to get. I should think that anything between 4,000 and 5,000 cubic inches would be fine. That'll give you enough room for three or four days of gear and food, without creating a monster pack that lets you pack TOO much. REI's Morning Star 75 ($145; www.rei.com) is at the high end of that range, coming with a decent suspension built around dual aluminum stays and good gear-organizing design. The Alps Mountaineering Red Tail is about the same size, sells for $170, and has a wide range of belt and shoulder strap options (www.alpsmountaineering.com). It's a good-looking, cleanly designed pack. Kelty's Tornado 4000 is at the low end of the same size range, and sells for $150 (www.kelty.com).
You might also look at Granite Gear's Nimbus Ozone 3800, a new, well-regarded pack that sells for $195 and features just under 4,000 cubic inches of capacity (www.granitegear.com). It has excellent suspension for a pack in this price range, so you could really overstuff the thing and still carry it comfortably.
Have a great summer!