it's not exactly a quick walk from the airport to downtown / pioneer square / seattle center
the quickest way would be to take a cab - the bus is cheap, but not something I would want to depend on given such a tight schedule
the light rail is starting this week and you could take the shuttle from SeaTac to the Tukwila station and take that into seattle
However, with only 5 hours to get off the plane, get into the city, do something and then get back in enough time to clear security and comfortably get to your gate - i'd probably spring for the cab or maybe spring for the Gray Line bus which runs to hotels on a fixed schedule
the Pyramid Brewhouse is near the stadiums and it would be closest to the airport - if it were me I would probably take a cab to the public market, walk around the waterfront back towards pioneer square & the stadiums, have a couple at pyramid and then make my way back
EMP is over in the seattle center area which is on the other side of downtown - wouldn't add on a ton of time getting over there if going by cab - but maybe an extra 30 minutes total and you may end up feeling rushed ........... however, it is over by the space needle, monorail and other touristy stuff
either way have fun and you should have a blast in vancouver