I'm not from Juneau, but I grew up a little south of that in Metlakatla. Be prepared for mosquitos; they are thick and persistent, so I always pack a headnet and wear it whenever I can. It can also rain any day of the year, and I've had August weather hit freezing in the mountains and 90 at sea level. Also be sure that your shelter can hold up in the wind... summer weather is usually pretty good, but you never know.
What else... bear spray is good, and I usually use a bear canister even when they're not legally required. Been meaning to try one of those kevlar bear bags instead, as canisters are big & heavy. I don't trust regular hanging bags because there's no place to hang them when you're above the treeline, and then the rodents will chew through and eat your crackers. Been there, done that. :D
Other preparations depend on how you get to where you're going. If you charter a boat or plane to get to & from your hike or to drop you off with your kayak, you'll want to stash at least a week's worth of supplies at your pickup point in case they're delayed by weather. For off-trail hikes, be really conservative about your mileage & pack extra food; the brush is really thick and is quite slow to swim through. Trail hikes are easy.
Oh yeah, the scenery and wildlife are awesome. Take a camera and have fun! I'd recommend seeing the Misty Fjords near Ketchikan, if you make it that far south.