For natural wonders between Astoria and Florence, I would suggest starting at Ecola State Park near Cannon Beach. There are wonderful views of the coastline including Oregon’s iconic Haystack Rock and the offshore Tillamook Rock Lighthouse. You can visit Haystack Rock by parking in Cannon Beach and if you visit at low tide you can explore its tidepools. Between April and July, you can observe Tufted Puffins nesting on Haystack Rock. About 15 minutes south, the highway climbs Neahkahnie Mountain and there are dramatic roadside viewpoints. I would then suggest following the Three Capes Scenic Route southwest of Tillamook. Cape Meares offers great views and a short walk to a lighthouse. The next stop is Cape Lookout which offers beach access and hiking. It requires more time to explore, but could be a nice stop for a picnic. Cape Kiwanda is the last stop and it offers great views if you climb to the top of the giant sand dune on its flank.
Other great stops as you continue south include Devils Punchbowl at Otter Rock, a great viewpoint and interesting geological formation. Yaquina Head Oustanding Natural Area is another must see, with a beautiful lighthouse, views and a great intertidal area if you visit at low tide. I would save plenty of time for the Cape Perpetua Scenic Area just south of Yachats. There are roadside viewpoints of interesting rock formations including Spouting Horn, Thor’s Well, Devils Churn and Cooks Chasm. The visitors center offers views and access to easy rainforest hiking trails. Heceta Head Lighthouse also makes a good stop with a short walk the the lighthouse in a spectacular setter. Highway viewpoints just to the south offer good views looking back at the lighthouse. Sea Lion Caves is just around the corner from there and offers more great views and access to America’s largest sea cave that is shelter for wild sea lions.
Just south of Florence, you will find companies that offer dune buggy rentals and tours. Honeyman State Park is a great stop for getting out to climb the dunes and the Oregon Dunes Overlook, a little further south, offers views and dune access too.