Have you taken a look at this map?
http://www.oregondiscgolf.com/
I'd say take a look at that and then cross reference them with the reviews here.
If I was planning a trip for an out-of-towner I would do:
Day 1:
Leverich and then Pier. Leverich is a great beginner to intermediate 12 hole course right over the river in Washington. It would be a great warm up for Pier which is considerably longer and more difficult. Both courses can get busy on weekends.
Day 2:
Horning's Hideout. The Canyon Course and Meadow Ridge courses are awesome. Highlands will definitely practice your approach and putting game, but I found it a little boring.
Day 3:
Milo and Timber. Its a bit of a drive (a little over an hour) but well worth it. I've only played each course once when I was really new so they kicked my ass but I'm looking forward to playing them again. I know they pull baskets in the rainy season but I'm guessing they would be back by then. The guys over at
www.nwdiscgolfnews.com will know for sure if you want to check before heading out there.
Day 4:
The gorge - Rooster Rock East/West and Dabney. I'm not the biggest fan of Rooster Rock but thats probably because I've only played the more open course on windy days and the wooded course tends to eat my discs, but combined with Dabney, they form a pretty good day of disc golf.
Obviously you can mix up those days in any way you may prefer.
On top of those there are quite a few small courses that aren't really worth going to such as Greenway, Orchard, Lunchtime, Rockwood, Vance, etc. There's nothing wrong with these and I enjoy playing them, there is just nothing special and if you're on limited time it would be better spent hitting the bigger courses.
I haven't played any of the courses south of here such as Champoeg, Ewin Young, and Hoover but they are next on my list to try. I can see how those would make a good day trip. I'll leave any input on those to others.
Disclaimer - I'm still a noob noodle arm who has lived here and played less than a year so the more advanced players may have a different opinion.