The Hoh RF visitor center is 5 hours from Seattle. In March, you could face one of the following scenarios:
1. Heavy rain but passable, muddy trails.
2. Heavy rain and trails that have become little streams. This happened to me one year. The trails were virtually impassable, and I had to turn back.
3. Gray but clear weather, possible drizzle with sun breaks, muddy trails.
The shortest trail is the Hall of Mosses; it takes 20 minutes round trip. This trail does not offer any glimpses of the Hoh River, but it does pass by some magnificent, mossy trees. You could also walk along the River Trail which has the advantage of offering several points at which you can access the Hoh, a gorgeous river with an unusual, milky blue color due to suspended mineral particles. The color might be obscured somewhat in a heavy rain, however.
Only you can decide if 10 hours driving for a short hike is worth it. Be prepared with mud resistant footwear.
Another possibility would be Hurricane Ridge, the summit of which is 3 hour and 45 minutes from Seattle. However, HR is only open Friday through Sunday in March, and you are required to carry chains in your car. As of today, there are over 8 feet of snow up there, with more all but certain between now and March. The snow shoeing is wonderful up there, and HR summit is often sunny even when it is cloudy at sea level. The visitor center rents snow shoes. Only you know if this is something you would want to do, but this year I am urging people to take the tire chain rule seriously. The park service has already had to close the road unexpectedly due to snow making the road dangerous.
I do not recommend the beaches in March unless we have an unexpected spell of dry weather. Windy March is my least favorite time to visit the Pacific beaches, and I usually stay away from them until later in the spring. It is very difficult to enjoy either Rialto Beach or Second Beach on a windy, wet day both because of the discomfort and because the views out to see can be obscured in the rain. Also, Second Beach, although beautiful, has a large driftwood barrier between the trail and the entrance to the beach. You have to walk on logs to get over this barrier, and the logs are dangerously slippery when they are wet. Rialto Beach is much more accessible. These beaches are 4 hours and 15 minutes from Seattle.