At the top of my list would be Ecola State Park, just 15 minutes south of Seaside. It offers easy walking paths with beautiful coastal views of Cannon Beach, Haystack Rock and other sea stacks, the Tillamook Rock Lighthouse and the capes and headlands miles to the south. It can be a great bird-watching location, too. In September, seabirds will be gone from their rocky nesting grounds, but Brown Pelicans, cormorants, bald eagles and many varieties of woodland birds are common. Be sure to visit the Indian Beach portion of the park, too. It offers some great views of interesting rock formations.
Haystack Rock on Cannon Beach is another must. Again, you will have missed seabirds, like the small colony of Tufted Puffins that nests there, but there are always a variety of birds and many photo opportunities. Plan your trip for a low tide to explore the tidepools. The little town of Cannon Beach is also very picturesque and fun to explore. It is known as one of the Northwest’s top art towns with several galleries and upscale shops and restaurants. Cannon Beach also has a short nature trail that skirts Ecola Creek, a good spot to look for birds and river otters. The nature trail passes the city’s water treatment lagoons that are also good bird-watching locations. Keep your eye out for herds of Roosevelt Elk in these areas; they are commonly sighted this time of year.
There are some nice areas just south of Cannon Beach you will want to explore. There are nice highway side viewpoints just south of town and some State Parks you should consider. I like Arcadia Beach and Hug Point for photography. Both offer access to beaches that are worth exploring. If you avoid high tide at Hug Point, you can walk to the north around the point to a picturesque small waterfall carved out of sandstone. Further south is Oswald West State Park that offers a few options for short walks, as well as longer hikes. The walk from the main parking area to Short Sand Beach is an easy one through rainforest to the beautiful, small cove beach. Further south down the highway, a small gravel parking area offers a short walk to Devil’s Cauldron, a basin of turbulent wave action at the bottom of sheer cliffs. A little further south are several turnouts along the edge of Neahkahnie Mountain offer expansive view of the coast to the south.
For longer day trips south, I most enjoy the Three Capes Scenic Route. The most rewarding location for a short visit is Cape Meares that offers a short trail that circles the edge of the cape, emerging at a lighthouse. You can make a full day of this and also visit Cape Lookout State Park and Cape Kiwanda, then return to Seaside via the US 101.
For day trips north, my favorite stop for photography would be Fort Stevens State Park where the 1906 shipwreck Peter Iredale can still be seen on the beach. The South Jetty area of the park is known as a good bird watching location.
During your time in Seaside, you will want to visit the estuary just north of downtown. A small park offers easy trails and often good bird watching.