Hello Ken,
You don't say how long you will be visiting Oregon, but here are a few thoughts. Driving north along the coast, it would be difficult to avoid finding great photo opportunities. There are lots of great rocky headlands and seastacks in a number of areas. Of course, there is a great element of luck in being in a particular place with the right combination of light and weather, so the longer you have to spend along the coast the better. Make sure you spend some time looking at tidal pools up close with various forms of sea life. There are several lighthouses along the coast--a quick Google search will give you a lot of info. Newport and Astoria (and a couple other places) have a lot of interesting old wharves, fish processing factories, etc. Astoria probably has the most history of anywhere along the coast.
For waterfalls, of course, the place to go is the Columbia River Gorge. There is lots of info available online. If you just have a short time available, you can see a number of waterfalls in a day or so without walking more than a couple hundred yards from your car. If you really want to experience the Columbia Gorge, you need to hike. There are many books of hiking trails available online and in stores in Oregon.
April will likely have changeable weather. Temperatures should be moderate. Dramatic clouds, rainbows, etc. are possible but you also have to be prepared for showers and hail.
