Seconding what HNL
said - SEA
(on either a -200 or -400), left hand side. If it's clear you'll get an awesome view of Ranier as you climb out, then St. Helens then Hood.
Anything that takes you over the Grand Canyon, Lake Mead & Lake Powell or Southern Utah. The Rockies are awesome as well, but you won't see as much detail in the winter, for obvious reasons.
The Pacific coast is always great.
And of course Hawaii, but I'd take a helicopter tour vs. a commercial flight.