I had several people ask me to summarize the Cascade Locks ride
suggestions for the list.

This map covers all or most of it (Thanks Del!)
http://www.skamania.com/pdf/biking-guide.pdf

Several people suggested a round trip eastbound through Washington and
westbound through Oregon. That is covered here:

http://web.cecs.pdx.edu/~black/Bikes/34%20Bridge%20of%20Gods%20map.jpg
(thanks Casey)

The best Oregon (eastbound) option is summarized here (thanks Aaron):

You can take Old HWY and a couple of frontage roads about 8-10 miles
past Multnomah Falls to Bonneville. Then you have to get on I-84 for
about a 1/2-1 mile (at most) section on that is not bad at all, large
shoulder, then you bounce off the freeway at Eagle Creek Fish Hatchery
and there is a bike/hiking path that takes you the rest of the way to
Cascade locks, maybe another 3-4 miles

Several people warned of the hazards of crossing the Bridge of the Gods
in Cascade Locks due to metal grating. Others said they ride it all the
time. There is a $0.50 fee for bikes to cross it.

Thanks again for all of the input.