Late July in and around Petersburg will be prime pink salmon and chum salmon fishing along with Dolly Varden. Unlike pinks you find a hundred miles up a river, the pinks right at the river mouth are good fun and still pull pretty well. The chum are usually more spawned out.
Bring a 5 or 6 weight to fish for Dollies eating eggs They are very fun. Petersburg also has quite a few cutthroat trout.
So even if it isn't primarily a fly fishing trip, bring your rods! Let me know if you have any other questions.