Sounds like a great day!
1. Yes the Malle boutique is a nice way to experience the line away from the Barneys hustle bustle.
2. The Diptyque store there will also have some scents and candles you can't find anywhere else so look out for some unique ones.
3. MiN and Osswald are both great, different, but great in their own way. At MiN try out their new self-branded line, also look for old stock in some lines and their shaving stuff. Osswald, see if they'll let you sniff...