What's inside Hachikuji's bag? In the anime, Hachikuji always carries a large backpack everywhere. What's in the bag? And why she always carrying it? Did she also carry it in the LN?

(I only watched until episode 5 in Monogatari Series: Second Season, so I don't know if there will be any explanation about this.)
Yes, she also has the backpack in the Light Novel. When she decided to visit her mom after a long time, she put memories in there to make her mother happy.

Here's a quote from the Mayoi Snail Arc in the LN (BakaTsuki translation):


  She neatly bound her own hair and filled her favorite backpack full of old memories hoping to delight her mother with them.  


.


   In the end she died with her backpack on, so she still carries it after her death.