As a quick guide to unlock everything though-
Skills
Tuesday: Talk to the Ninja behind the Library's atom sign in the North East of town.
Unlocks Shadow Clones (Make twice as much money when working)
Wednesday: Go to mall, walk to drug store, get cold medicine from the middle square of the middle shelf on the LEFT side of the store. (Two squares right and four squares off of the purple girl next to the freezers) Then give cold medicine to the sick ninja/man in the top right of the same store.
Unlocks Daily Training (Gives double stat upgrade when working)
Thursday: Find and talk to the cat hiding in the trees directly south of the convenience store.
Unlocks Fast Resting Yoga. (Nap for 1 AP to recover stamina, scales with stamina recovery gain from working bath house)
Friday: Find and talk to the invisible person inside the disconnected wall South East of the bath house. (Just spam interact on different parts of the wall to find them)
Unlocks The Meaning of Labor (More likely to succeed while working)
Saturday: Talk to the mumbling old man in the book store 10 times in a row in the mall.
Unlocks Sex and Life. (Increases Max Lust? A bit fuzzy on the details, but that's mainly it I think? It says double max lust, but it apparently doesn't apply to jobs, and also I didn't see much difference. My lust was 1,181, it went to 1,193 after I unlocked it? Don't know if bug or if it's meant to double your CURRENT lust when having sex, as in lowering lust requirements for actions effectively. Either way, that's how you get it.)
Work Gifts
Work each job (bath, construction, convenience store) 15 times for a stat boost.
Infinity Elixir: Get every achievement in the game and then go to the library after the ending. (Restores all action points, refills stamina, and increases Lust to Maximum)
Rest of the game items can be bought at a shop after your first playthrough with more items unlocking as you complete more achievements.