Can you get "dropped off" elsewhere? I'm not sure what kind of a tour you're on, but if it's a cruise I'd just fund my own transportation instead of going to SeaTac 8 hours early. You have at least 6 hours to spend before you need to arrive, and at 9:30 pm SeaTac is almost deserted.
The biggest problem is that SeaTac is not in Seattle or Tacoma, and there's not a lot to do in the immediate area. If you're already in Seattle I'd stay there and head to the airport around 7. There's a lot you can do in Seattle in 6 hours. Likewise if you're tour ends in Tacoma (which I really don't expect) I'd follow the same time frame.
If you do have to go there first I'd check my baggage and depart. (Note most airlines do not want you to check baggage more than 2 hours early.) You can rent a car, Uber, Lyft, take a cab, etc; but don't hang around the airport.
Southcenter Mall is 2 miles from the airport. Lots of stores, movie theaters, places to eat, etc. A much better option for just killing 6 hours.
If you're interested in airplanes take a ride over to the Boeing Museum. You can easily spend 3-4 hours there.
If you like gardens visit Highline Gardens and walk around a bit.
13 Coins is okay, I've eaten there dozens of times when waiting to pick someone up at the airport or for business. But you have enough time to do better. Take a ride to Burian. Eat at the Elliot Bay Brewhouse or the Burian Fish House. Then walk a block to Bakery Nouveau and hope they still have something for sale (very possibly the best bakery in WA State.) There's lot's of little shops if you want to walk around there too. But even if you head back to the airport it's still going to have been better than just sitting in an uncomfortable chair and paying crazy prices.