Guys have given some good travel tips so far. In the Columbia Basin, (along with the wheat harvest and the little hills the guys cut wheat on) You might run across hay baling as the basin gets 4-5-6 cuttings a year.  (won't actually get to see alot of actual baling unless you want to be driving around at in the middle of the night literaly. They bale at night as there is almost no humidity compared to what you are probably used to) You'll probably see some wheat combining in the irrigated basin where 130-150 bu/ac is more the norm.

Grand Coulee Dam has a good tour and nightly lazer light show during the time you'll be here. Explains how the dam is the start point for the entire basin, background of the area,etc. there's also an interpertive center explaining the dam etc.  Depending how you'd drive to Grand Coulee, you would be driving through about the heart of the Columbia basin, and get to see some a plethora of irrigation systems, and farms.

Hell's canyon tours runs jet boats up the river between Wa,Id,Or, out of Clarkston/Lewiston 100 mi. or so run between Idaho and Oregon.   (kinda like Rogue River run)   

Better question would be,,"what don't you want to see?"   North out Quincy Wa, driving toward G.C.D.  there is a interpertive center overlook explaining how when "lake Missoula" drained,, it was something like 5 times the size of Niagra Falls. (all kinda looks like scab ground now, but still lots of recreational activities in the area,etc.

Pretty much depends on what you want to see, and how much time you want to spend.