It's the US - it's easy to get here. SeaTac (SEA) is our main airport with international connections to virtually everywhere. You'll need a car while you're in town and you should be warned that traffic is nearly as bad as it gets in the US. But the drivers are about as polite as they come and it's pretty easy finding your way around.
You can get to Seattle by Amtrak trains which drop you off in the International District where you can catch any number of buses in the bus tunnel.
Washington also has a great ferry system. You can catch ferries to Canada (Victoria on Vancouver Island) or all the way to Alaska.