Do they even have the large, oblong watermelons in Japan? All of the ones I see in Japanese media are round ones so small I’d almost not conciser one worth bending over to pick up.
So reading the various suggestions and thinking about what would be novel, I’m thinking the way to pack it all in to a couple of days is to load your truck up with tents, steaks, guns, and shooting cans and head on down to somewhere in the country.
Go mudding in the country. Have the kids shoot cans. Cook em up some steaks and baked beans out of a redneck barrel, and then sleep in tents. Yeah buddy, that’s the life.
Friend, I don’t know about you, but where I live, even organic Honeycrisp apples don’t cost $7 a lb. 
Heh. I do this every few years with a group of friends. We add beer and weed to our activities and call it Men’s Town.
Try to teach them the tipping rules. Especially cow-tipping. Also, chick-cruising, ham-boning, heavy-petting, break-dancing, off the top of my head (or their head?).
Japan is all about regional food specialties, so if you could explain what Seattle’s specialty is (coffee?) - and teach them how to tell a good one from a bad one, etc., that might be interesting for them.