dkuntz,
Look here for info on all the state parks
Oregon State Parks Each park will have info and a link to brochures & campground maps.
My thoughts on best campgrounds for biking & exploring from north to south:
Fort Stevens State Park (Astoria/Warrenton) - miles of bike trails, tons of history, freshwater lake for fishing/swimming, near Astoria and Lewis & Clark's Fort Clatsop.
Nehalem Bay State Park (north of Tillamook) - on a large peninsula, you have the ocean on one side and a large bay with great gently sloping beach on the other, kayak in the estuary in the bay; miles of biking including back road to nearby tourist town (Manzanita).
South Beach State Park (Newport) - lots of beach
Jessie M Honeyman Memorial State Park (Florence) - sand dunes!
Bullards Beach State Park (Bandon) - day use/beach access is about 2 miles inside the park from the campground (good bike ride), easy beach access.