I just found out there is a zipline in Stevenson, 45 minutes east of Portland.
http://zipnskamania.com/http://www.skamania.com/columbia-river-gorge/zip-line-tours.phpThe information on the two web pages is quite different so I don't know which is more accurate. However, it is impossible for a person alone to do this because depending upon which page one believes it either requires a minimum of 2 people or 4 people to book a zip trip.
Anyway I through it out as a possible UTS adventure. It's not cheap, but it's not the most expensive of these I've seen either.