Alaska Airlines and Delta have round-trip flights from Phoenix (PHX) to Seattle (SEA) for $118-$128, NONSTOP. Flights October to March. No Sunday departures. Weekends available. Origin/Destination reversible.
A Carry-on bag and also a personal item is allowed for free on Delta. You can avoid paying for seat selection by accepting the randomly assigned seat at the gate.
Booking a Flight+Hotel together as a package will usually be cheaper than booking each one separately. You can easily construct your own package deals on Expedia, Travelocity, and Priceline. The savings amount will vary depending on the combination of website, flight, and hotel, so be sure to compare the checkout prices on each site before purchase.