There's a variety of interesting places worth visiting around the state:
Portland Japanese Garden If you can visit here in October at the peak of the fall colors you can photograph the famous Japanese Maple. You will see many people surrounding it, waiting their turn to get below the canopy to get the classic shot. Best light is usually mid-afternoon with the sun behind the tree.
John Day Fossil Beds National Monument There are three separate sections to this site but the Painted Hills Section is the most photogenic.
Smith Rock State Park north of Bend is a good sunset location
Columbia River Gorge There is a great view of the river from the Vista House. Some of the best places to visit and photograph are:
The Lavender Valley near Mt. Hood is a good place to photograph the farms with rows of lavender and Mt. Hood in the background.
Tulip Fields Best photographed in the spring when the tulips are in bloom.
Wooden Shoe Tulip Farm - Woodburn
Vanderzandern Farms - Hillsboro
Silver Falls State Park Hike the trail of ten falls. One of the better places to see waterfalls including South Falls where you can walk behind the waterfall to photograph it.
Thor's Well Along the coast is a large depression in the rock shelf that creates a series of waterfalls as the waves break over it and the water gets sucked in.
Crater Lake National Park In the far south of the state its worth staying at the park lodge right on the rim of the crater. Good views of the stars at night as the park is far from any major city.