3 days in Pullman eh? If you haven't been through Hells Canyon, then definitely consider taking a long day and run through there. If you are up for a really long day (600 miler), you could make a loop out of it by running south on 195 through Lewiston to 129 (3 in Oregon), to Enterprise.
There is a NFS road from there over to Hells Canyon, but I would check with the locals in Enterprise to determine if it's S/T worthy. As I recall, someone told me it's gravel (not sure though).
The known paved route would take you west, around the mountains to Baker City, then east again to 86, 86 all the way through Hells Canyon to ID 71, then 95 north all the way back to Lewiston. Lots and Lots AND LOTS of Canyon Carving! You might be dizzy after that ride.
If you just want to see Hells Canyon, then just do the 95/71 route down and back.
Another great run (but another 500 miler) would be 12 (east towards Missoula MT and back). There's also 11 (off of 12, going north) that I've been wanting to see, but again, check with the locals on it's condition. That section of 12 is the highway with the sign “Curves ahead next 99 miles”.
You're also pretty close to the Grand Coulee Dam (155 miles to the north-west), and according to BDF (Brian), definitely worth seeing. It's also on my bucket list.