I'll be visiting Seattle with a friend next month. We'll be flying into SEA and traveling light. Best way to get to downtown from there?
Link Light Rail! For less than $2 (last I rode it in 2012, it was $1.75), you can ride the train from SEA to downtown, with several stations there's probably one really close to where you're heading. Just follow the signs at SEA to the train station/Link Light Rail, which is located right next to the fourth floor of the airport garage. Trains run from 5 AM to 1 AM on Monday through Saturday, and 6 AM to midnight on Sundays, and they run every 7.5 to 15 minutes, depending on time of day.
There's also a bus that goes to downtown and here's more information about it from the SEA website:
"The Metro bus stops that serve Sea-Tac Airport are located on International Boulevard (State Highway-99) and South 176th Street by the Link Light Rail Station. The most convenient route to the stops is to take the walkway through the north end of the fourth floor of the Airport Parking Garage to the Sound Transit Link Light Rail Station and exit to International Boulevard.
Departure times are shown on information signs at the bus stop. You also can pick up printed bus timetables at the Ground Transportation Information Booth on the Baggage Claim level near door number 16 (across from baggage carousel 12)."
Here's a link to more specific information about actual departure times and lines to and from SEA: http://metro.kingcounty.gov/tops/bus/destinations/flymetro.html
If you get in late & the Light Rail is closed for the night, I highly suggest Uber. Download the app & once you get close to exits, request a car (black car if you're feeling fancy, UberX if you want to save a few bucks). They have flat rates between downtown and the airport that are comparable to a cab (or cheaper with the X) and you don't have to tip/pay in the car. (All payments are handled through the app & a card you put on file.)