All items for 100 rupees:

Go to Rennu Village (Past) and find the 
locked door next to the Target Aim mini-game.
It can now be opened, allowing access to a 
shop that sells all items for 100 rupees.


Harp of ages:

After you beat the first level, go to Nayru's
house. Talk to Impa and then push aside the 
block that's in front of her. Go inside the 
room behind the block to get the harp.