Not sure what lenses you're plan on taking, your profile is not filled out. Plenty of landscape/macro pictures around Mt Rainier (2 hours south of seattle). Snoqualmie falls is near by and nice city skyline from West Seattle. If your staying in down town seattle be sure to head to the pier and get pictures of the olympic mountains with the ferries heading to the penisula.
As much as I love to watch whales, I would discourage anyone from going on an Orca watching cruise in Puget Sound. The Orcas are having a very hard time finding enough food in the past few years. One pod seems to have lost several members this past year to starvation. Orcas find their food with echo-location, and the engine noise from the whale watching boats coming out of the US and Canada make it difficult for them to find food. At times you can see a pod surrounded by small whale watching cruises.
Jim Kelly There's no sense making a mistake unless you plan to learn from it.
If you go to Rainier plan to dedicate an entire day to the adventure, and try to get there before the sun. In fact, you may want to go down and stay at Paradise Inn or at one of the bed-and-breakfasts near the park so you can get a jump on the day. Paradise has spectacular, up-close, views of the glaciated face of Rainier. Sunrise is, as the name suggests, an awesome place at sunrise, but I'm not sure if the road to Sunrise will be open in May (it might still be covered in snow).
The entire coastline of the Olympic peninsula is good.
If you come in May I think you will have just missed the Tulips in Skagit Valley. But if you find yourself coming in April instead of May, the tulips will be exploding in colors.
Many people find a ferry ride to be a good location for photography. You can take a ferry from Seattle to Bainbridge Island, spend an hour or two in Bainbridge, and then come back to Seattle. You get shots of Seattle and the Olympics from the water, and Bainbridge itself is quite a picturesque place. You don't need your car for this trip. But if you do bring a car you will want to spend more than a few hours. Just driving around the perimeter of Bainbridge Island (stopping at all the incredible vistas for more shots) could consume an entire day. I go on a bike ride there every February (the Chilly Hilly) and find myself thinking "I need to come do this loop with my camera instead of my bike" (here's a map of the route).
La Conner is a quaint fishing village/tourist trap about 1.5 hours north of Seattle.
Shillshole Marina is a great place for sunrise or sunset photography. You have all the boats of the marina, Seattle and Rainier off to the Southeast, and the Olympic mountains off to the Southwest. And if you happen to find yourself there in the evening, Palisade, one of Seattle's best restaurants, is right there.
Other Possibilities: Kerry Park - View from hill north of Seattle. Great shots of Space Needle with Rainier in background. Pike Place Market - Lively, colorful, open-air market. Famous fish-tossing fish sellers.