I highly recommend some or all of the "Totally Tublar" North of Seattle by the zoo area. They are very unique puzzles.
http://coord.info/GCP9G9
http://coord.info/GCNDTN (Currently disabled but may be fixed by your visit)
I haven't hit any others of the series, but if these two are the standard, the others will rock!.
Snoqualmie Falls Volcano
Depending on the setup the EarthCace at Snoqualmie is neat. Sometimes the hike down is not required if you can find the attributes need for the GC or if it is changed like it is now to not require the hike to find the items listed.
http://coord.info/GC20ANM