Just opposite Notre Dame, on the left bank, you'll find one of the most remarkable shops, and one of my favourite shops, in the whole world. It's a bookshop - to say more than that is pointless. If you like book shops just go and see it. Spend time there and absorb everything the place has to offer. Then go to the cafe next door and have some coffee...and mull over just how remarkable it is. Then let me know if you think it is too :)