The STP is a mostly drab route, but there are some changes you can make to the route to enhance it. Taking the reverse of the STP brings you along OR-30 and over the Longview bridge, which is not really a pleasant experience. OR-30 has a wide shoulder, but it is highway.
I suggest going over the I-205 or I-5 bridge into Vancouver and riding north on the WA side...Prepare for damp conditions that time of year and expect a prevailing northerly wind.
Second the STP routing. It's the most direct way between the two cities. As waynemyer states, it isn't a particularly thrilling route, but I don't think any direct bike route between Seattle and Portland is going to be more exciting. I like it o.k. but it's a mix of woods, farms, and small towns, at least until you reach Seattle metro area around Spanaway. (You really don't encounter Portland's suburban sprawl using US 30.) And it's fairly flat to rolling, with only a couple decent hills.
I rode the Longview (Lewis and Clark) Bridge once and wouldn't do it again. Long, steep, narrow, littered with logging debris. The routing to avoid it and get over the Washington side is here:
It's pretty low traffic and rolling for the most part, using quite a bit of old US 99. However, there is a monster of a hill north of Woodland, between mileposts 27 and 31 on the map. Since I-5 plowed over old US 99 here, this is the only way around it, unless you hop on I-5 for this section (yes, it's legal.)
As for wind, I've ridden northbound this way in May/June and didn't remember much of a headwind. The wind inland isn't as bad as the coast. Of course I say that now and the next time I ride this I'll get the fierce headwind.