Like a Dragon: Infinite Wealth

During Chapter 6, you'll be introduced to Dondoko Island by Matayoshi, and freakish mascots Gachapin and Mukku, which also ties in with substory 33. You'll then begin the task of clearing up the island, ready for guests. Once you've cleared enough of the island, you'll be able to build lodgings for said guests, then start inviting them over. Fulfilling guest criteria is important, and once you've reached the level of 3-star resort, you'll be able to build any kind of (from lowly F rank to swanky S rank) lodgings you like, Dokobucks notwithstanding. You can then start inviting loads of guests in one go, speeding this up.