It occurred to me to suggest the Village St Paul in the 4th as an unusual destination. This is a small collection of adjoining courtyards and passages with some interesting, mostly antique shops. It is quite quaint and charming.

For someone 12, staying near the Pompidou might be a lot of fun.

Finally, all the talk of chocolate has me thinking of two yummy chocolate destinations in the 7eme: the patisserie Dallyou makes something called Delice d' Pistache that is a sort of croissant with chocolate and pistachio paste that I think of very often; and Michel Chadon is a beautiful shop with little cubes of dark ganache that I also think of very often. ;-0

Bon voyage, Debbie in Pittsburgh