Go Take a Hike, Seattle
Luggage and bags:
Backpack or at least a daypack to go out hiking
Clothing/Shoes/Weather Gear: Hiking boots/shoes,dress shoes for night,though there are not that many places with strict dress codes.
Bring a sweater even in the summer and a coat in winter(gore-tex would work or just a medium winter coat)
Photo Equipment: Bring a camera for sure as there are so many photo opportunities.
Camping/Beach/Outdoor Gear: We are the home of REI as well as many other outdoor outfitters so if you need you can stock up here.
If going deep into the mountains prepare for cold as well as warm weather.
The beaches are beautiful but not very warm,usually breezy and cold at night
Miscellaneous: Though most beaches here are flat and sandy the NW part of the washington coast is very rugged with driftwood and perfect for a relaxing camping trip.
Take some good hiking boots! I used gaiters when I climbed St. Helens. They helped to keep the ash out of my boots. Depending on when you go you might need some crampons for hiking above the treeline.
Photo Equipment: Quality camera with a good zoom lens.
Luggage and bags: Since Seattle is such a great walking town I recommend a sturdy backpack with plenty of pockets for my digital camera and water bottle.