Get a large vacation house and hire Redneck Joe to cook for all of you while on vaca! His stuff looks mighty good!
Cooking for 20 for a week would be a tough one. I've done big one off for up to 75 folks and that left me drained.
I've looked at the train rides in the PNW but for you might be too long and grandkids in a small tube shaped vessel for more than a couple hours is .... we'll don't want to give you nightmares so I'll stop.
the opryland (or similar) would work however thinking on the incremental costs of food which in a tourist trap like that (and nashville in general) will add up on top of room cost. the cruise being all inclusive may be the best bet unless you can find one in the US mainland. another thing to consider is the cost of getting to your chosen place. there again a cruise out of FL usually has some good deals just to get you to the port. Speaking of good deals, make sure if flying you call the airline to get a group discount. a cruise also usually does not require passport for day trips.
a few years back AC and i did a trip to jamacia. if you have ever seen Christmas with the Cranks where they try to skip Christmas... well we succeeded in skipping thanksgiving. we travelled on thanksgiving day and the bonus there was the fare on that day was stupid cheap and the airports stupid empty. The return fare and airport volume was more but still light as so many people had just travelled for the holiday. Mostly business travellers (we can spot each other) so things went more smoothly thru security, boarding, etc. we did an all inclusive Hilton property. I even bucked up for the all you can drink and a persona butler. the butler was fun. you get a walkie talkie and a channel and boom.... service - even having reserved spots on the private (and small) beach. They had put in rock barriers to keep that beach's water nice and calm for snorkeling. fun story on the all you can drink. they were only allowed to bring 6 beers per person in a trip, kinda like olive garden on breadsticks i guess. The good stuff (rum vodka and whiskey) were in something like an inverted soap dispenser - just walk up and fill your glass. after the first couple days we got some slight pushback from our butlers (we had three to cover the 24 hours) about how much we were going thru to which i slightly more strongly pushed back to him. we won.
so that last part was to circle back to a cruise. I really think that is your best option for ease, food and passport issue which you are right just took my wife about 5 months to get her renewal - but if you (flight) travel on thanksgiving day the cost should be way lower. Depending on the cruise departure timing, for 20 people you can get a discount on a hotel for a day or two if needed. Look for well reviewed local hotels - they will negotiate better especially if you tell them you won't need your room serviced during the couple days you are there. Then groups go out and do things daytime, everyone meets for supper - again calling to get group discounts where you've decided to eat. Then grab your cruise and go. if everyone played nice at the motel, they will be happy to store everyone again for a night if needed on return. All this and you've had a family thanksgiving that not many will forget.
lots o words there but at least you didn't have to pay for any of them. Most importantly please get those really dorky family vacation tshirts to wear for the whole trip.
Embrace the your Griswald....