Off busy Highway 6 on the way to Tillamook and the Oregon coast, Elk Creek Campground is a sweet getaway from a lot of busier campgrounds and destinations in this region of Oregon (i.e., an hour from Portland). It's situated near the meandering and scenic Wilson River, which offers numerous floating, swimming and diving opportunities.
The campground itself is nestled in the forest with plenty of shade on warm days. It's pretty no-frills, which certainly isn't a bad thing when you're spending your time exploring the numerous trails, playing in the river, visiting the nearby Tillamook Forest Center or grabbing a pint and bite in fairly sleepy Tillamook.
Mountain biking—some really good stuff—is very close at hand, as well as hiking the Wilson River Trail or ascending Elk and Kings mountains (steep scrambles up and down in many places that I wouldn't recommend at all for small kids or those not looking to bust their lungs or stand on vertiginous precipices!). My Merry Wanderers crew actually busted out the Elk Mountain-Elk Creek loop straight out of the campground, covering ~9 miles with ~3,890 feet cumulative elevation gain in ~4 1/2 hours—it was exceptional and comes highly recommended. Next time we plan to add the connection to the Kings Mountain summit.
Overall, a great spot in the woods with plenty of playtime options right out your tent!
Near Lake Cushman and not far out of Hoodsport off Highway 101, this easily accessible campground is away from the summer craziness that afflicts so many campgrounds in the Olympics. That said, it's still very close to the big boat-friendly lake (just a few miles to the south) and Staircase, which is an amazing entry point into Olympic National Park.
The surrounding forest is lush and gorgeous, the campsites fairly spread out (there are 23 sites for tents, trailers and RVs, with the usual amenities) and as an entry point to outstanding hiking it's first rate. My crew hit the short trail that circumnavigates the campground and then hooked up to the Upper Big Creek Loop Trail, which climbs into the forest, features some nice bridge crossings and affords views of Mount Ellinor (you can connect to the Mount Ellinor Trail, too, if you're super ambitious and ready to go big!). Big Creek roars steeply down through part of the trail (under a few bridges, that is!) and then skirts its namesake campground.
Big Creek Campground is definitely a stellar basecamp for exploring the south Olympics area, which abounds with trails, climbs and fishing spots, and I'd highly recommend you giving it a shot.