First, welcome to the ACA Forums!!
Your question is a bit difficult to answer since you don't say where the start of the tour is or if you mean home.
Assuming you rode the Northern Tier, I would probably ride to Portland, ME (about a 3 day ride) and then take an Amtrak assuming you could do a roll-on or curb check (NOT boxed) bike add-on to Seattle. Good part is that it allows you to partly retrace your route and "decompress" on the long 81+ hour ride back to Seattle.
Option two would be to ride to Portland and drop the bike off at a bike shop and have them ship it home while you fly back to Seattle. A lot quicker but more expensive and better chance of the bike getting damaged.
Option three would be to ride to Portland (see a theme here) and rent a car one-way. This allows you the most flexibility and safety of the bike but the cost is by far the highest and it about 3200 miles one-way assuming no major detours.
Option four would be to ride to Portland and take the dreaded Greyhound back to Seattle. This is by far the worst option to me. It takes 7 transfers and 78 hours and is more expensive than the train which to me would be much nicer.
Of course, you could ride your bike back but then I doubt you would be asking this question if that was a viable answer.
Basically, the 4 options could apply to any ending point so long as the option is available, i.e. Amtrak or a car rental agency that allows one-way rentals is nearby. It basically boils down to your preference and budget.
Hope you have a wonderful trip! Tailwinds, John