After a meeting in Portland in early June, I have just a few days to drive to Bellingham; I'd like to take the coastal route and am looking for suggestions about what not to miss, where to stay if I decide to break up the trip (although it's only about 4 hours, right?)...

At least four hours - and that's if you take I-5 straight not the "coastal route." Do a quick google maps routing of directions between Portland and Bellingham: the "coastal route" doesn't make a lot of sense - it will add hours to your route if you take US 101 from Seaside north back up to Olympia, WA (back to I-5 the rest of the way) and you don't get a whole lot of scenery that way anyway.

More like 5 - 5 1/2, counting traffic through the Seattle area.

By "coastal" do you mean Puget Sound or Pacific Ocean?

Several attractive alternatives, depending on interests and time:

West from Portland down the Columbia River (WA side past Longview) to the Long Beach Peninsula/Willapa Bay - good lighthouses at Ilwaco, cute town at Oysterville, then north along 101 to Lake Quinault and Hoh rain forest valleys in Olympic NP, several great wild beaches (Ruby Beach, Rialto Beach, numerous others.) Then up and around the top of the Olympic Peninsula (optional day trip to Victoria BC by ferry from Port Angeles) then ferry from Port Townsend to Whidbey Island and SR 20 to SR 11 (Chuckanut Drive, scenic coastal road) or I-5 into Bellingham.

Or, I-5 north past Mt. St. Helens to Olympia, then 101 up the Hood Canal to Port Townsend, then over to Whidbey as before (or side trip to Port Angeles/Hurricane Ridge plus Victoria as before)...

Or (you'd need a couple/3 days,) east from Portland through the Columbia Gorge (Oregon side past Hood River, use the historic highway, not I-84 in Oregon) to US 97 (stop at Maryhill Museum if time allows) then north through Yakima to Wenatchee - beautiful orchard country on the east slopes of the Cascades. Then west on US 2 over Stevens Pass to I-5, west on SR 526 to Mukilteo for the Whidbey Island ferry, then up Whidbey, over Deception Pass, and back on SR 20 (as above) to Chuckanut or I-5 into Bellingham.

Lots of options involving mountains, ocean, big rivers, rain forests, inland waters, old west scenery - you name it.

You say you have "a few days" to drive from Portland to Bellingham, but it only takes a few hours to make that trip.

If you are asking what to see during a few days after your meeting, I suggest the Columbia River Gorge, the Oregon Coast, Mt. Rainier, and Seattle--depending on how many days you have.

There are very few opportunities to view the ocean while driving in Washington State. Three of my favorite drives with water views in Washington are Highway 112 between Clallam Bay and Neah Bay on the Strait of Juan de Fuca, Highway 101 along the south shore of Lake Crescent in the Olympic National Park, and Highway 101 between Seal Rock and Hoodsport on Hood Canal.