You used to be able to get into the ARTCC (Air Route Traffic Control Center) down in Auburn just by calling for an appointment. That was many years before 9/11. You would be escorted inside the building and, if their workload allowed, an air traffic controller would give you an informal tour and allow you to stand and watch the controllers at work for a few minutes. Oddly, it's a solid concrete building with no windows. The tower at SeaTac handles ground control (airplanes moving around on the taxiways), and arriving and departing traffic only. Once aircraft leave the control zone (a few miles from the airport) their control is turned over to the nearest ARTCC. I suggest you start by calling the regional FAA office and asking them about a tour. It may be helpful if you had a good reason for them to give you a tour: Have children? Can you somehow make it a school field trip? Know any boy or girl scouts? Think one of them would be interested in an aviation merit badge?