At the mouth of the Columbia River, which makes up the border between Oregon and Washington, you will find the Lewis and Clark National Park. The name is a misnomer - there are actually a number of parks and historical sites that make up the park.
Lewis and Clark crossed the continent and came to Cape Disappointment, which was named by British fur trader John Meares after a bad storm forced his ship to turn around without reaching Canada. The US Military created a fort there, later named Fort Canby, during the Civil War. Fort Canby was decommissioned after World War II, and the park was set up in the early 1950's.
Cape Disappointment State Park is the largest location in the Lewis and Clark National Park. It is over 1882 acres, includes two miles of shoreline, old growth forests, two lighthouses, the North Head Lighthouse and the Cape Disappointment Lighthouse, and a freshwater lake. There is also a large, recently renovated interpretive center.
There are miles of hiking trails at Cape Disappointment, as well as camping areas, which include yurts and log cabins. The campground borders on O'Neil Lake, which is a freshwater lake. The remains of Fort Canby, including the artillery battery, is still accessible and is a very easy hike.
Most of the trails in the park are at least well-graded, if not paved. You can see both lighthouses, as well as old growth trees and salt marshes. Leashed dogs are welcome throughout the park, particularly on the beach.
The beach at Cape Disappointment is broad and flat and well-populated, with kite flyers, clam diggers, and fishermen. You will find plenty of dogs, but not a lot of swimmers. The water is very cold and has a strong tide. Be prepared to use a lot of caution if you want to let your dog play in the water.