I liked Petersburg as much as its cousin down the way (Wrangell). The difference being that Petersburg seems to have more money (town economy) and more blondes. The downtown is quaint and fun, but if you can, drive around the area further out of town and you'll have a great time seeing the scenery and the Sitka deer.
Petersburg is a fishing town with a huge harbor lining its coast. There are some great kayaking and water adventures just outside of town. However, be sure to paddle with the tides or a one hour trip can turn into three ;)