I am a little confuse at how some of you are missing content but since I don't have good memory I don't remember how I unlock most stuff, so here's some tip:
1. Game is not completed even when MC say he has paid off all his current debt, he will continue paying a new debt for the studio rent, only then it's completed.
2. Some area is unlocked by interacting with object or NPC, talk to every NPC you can click on. If you have completed the game and still haven't unlock everything, complete REM quest and eventually you will unlock the aura eye thing and an event tracker which reveal all object and NPC that can be interacted. The event tracker will put an exclamation mark on any node that currently has an event you haven't found yet. 'Don't fully rely on it, go explore every node, some sex position unlock is not marked by event marker"
3. After unlocking the hot spring inn you will unlock Maiko continued quest, which require a voucher which you can get from luxury hotel.
4. When Rem gave you the secret invitation it might seems confusing on where to use it, just go to the club and call Nadoko and do a shooting there, you will see that a new shooting has been unlocked.