Consider Yachats as your destination. I think it fits your description and desires better than any town on the Central Oregon Coast. It’s a popular destination, but it’s definitely quieter than North Coast destinations like Seaside or Cannon Beach in mid-September. It’s just outside of the Cape Perpetua Scenic Area where you will find more secluded beaches and some of the best hiking on all of the Oregon Coast.
You will find a reasonable gentle hike between Cook’s Chasm (Spouting Horn and Thor’s Well) and Devil’s Churn or you can just explore the paved trails that extend from either the Cook’s Chasm and Devil’s Churn waysides. There are also a variety of rainforest trails including some shorter interpretive nature trails. A visit to Heceta Head Lighthouse, a little further south in Florence, is also a must and the short walk from the State Park to the lighthouse is beautiful. You could easily explore as far south as Florence and the Oregon Dunes, where Honeyman State Park makes an easy stop and offers nature trails around Cleawox Lake.