The season really affects your packing in Alaska. Keep in mind the sun almost never sets completely, so a hot day can stay hot at all hours. Yet, on the coast or on a boat ride, temperatures can dip down near freezing even when it's cold. Check the average temperatures for where you are going, but bring clothing suitable for all conditions even in summer.
The draw of Alaska is the wilderness. Even if you don't plan to hike you will likely spend a lot of time walking. Bring at least two pairs of comfortable, broken in walking shoes. Hiking boots are a must if you are hitting the trail. Good quality socks and blister pads are also a must have item in your bag.
Don't forget sunscreen. The long days and northern atmosphere make sunburn a real threat. Sunglasses are another item to pack, with a spare just in case. Mosquitoes are a real problem in some parts of the state so you want a good repellent on hand.
Finally, bring the gear you need for any special activities or make plans ahead for rentals. Your travel agent can help you perfect your packing list and guide you in your rental gear arrangements before you ever step foot on the plane.