Well this is a pretty unique offer, and quite exciting for the future of Switch Online. Nintendo is going to give Switch Online members in Japan the chance to enjoy Captain Toad: Treasure Tracker for free!
From Aug. 5th to 11th, 2019, anyone in Japan with a Switch Online account can grab a special download version of the game that gives access to the entire experience, and it can be played as much/little as you want during that 7-day period. After Aug. 11th, 2019, the download will cease to work. In other words, you can grab the full game and enjoy it from start to finish without paying anything more than a Switch Online membership! It'll just vanish after Aug. 11th, 2019, so be ready!
Thanks to NintenDaan for the heads up!