Bring rain jackets and waterproof shoes or sandals when you visit the Olympic peninsula. Also, a fleece layer, even in summer, is a good idea. Make sure to use bathrooms before heading to the beach. It is quite the hike to get back to them once you have descended the trails. Pack snacks and drinks for sure. Parts of Olympic National Park require a park pass, depending on where you enter.
Depending on where you want to stay in the area, you will need to research accommodations six months in advance. The sites at Kalaloch Campground are reserved almost immediately upon opening 6 months before your arrival date. This, or Kalaloch Lodge would be your best options to stay nearby. Olympic National Park does have first come, first served campgrounds elsewhere in the park.
If you are willing to wear your rain jackets (as you should in Olympic National Park), the beaches are actually quite scenic when there is a drizzle. Summer months have milder weather for exploring at a comfortable temperature.
If you have a child or immediate family member with a permanent disability, you can get a free National Parks and Federal Recreational Lands Access Pass. This is a lifetime pass that allows access for the pass holder and everyone in their vehicle or campsite. National Park campgrounds give half off the basic site fee (not including electricity fees, etc.) with this pass.