Luggage and bags:
Not unless you spend the night in Leavenworth, you don't need to pack your bags!
Clothing/Shoes/Weather Gear: Light clothes in the summer and dress up warmly during the winter time. Walking shoes.
Toiletries and Medical Supplies: Just the stuff that you need. There are pharmacies around and stores closeby
Photo Equipment: Your digital camera and your video
Camping/Beach/Outdoor Gear: Swim wear and life jackets for your children if you intend to go white water rafting
Whether skiiing, camping or hiking in Steven's Pass (just outside Leavenworth) it is cold!
Photo Equipment: Steven's Pass. Take a camera! You'll cross it travelling from the west side of Washington State (Seattle) to the East. Snow caps are lovely mostly year round!