Feitan
I have a headcanon where his 'Birthday' is celebrated on Christmas until they find out when his birthday really is, as Chrollo suggested.
You always buy presents for the Troupe, and the Troupe usually steals presents for each other. Present exchange.
They always steal one extra thing for Feitan to give to him during the exchange. He always says that they don't have to, until it comes to you giving him a present. He'll thank you and give you a hug.
Feitan will get sad if anyone forgets his 'birthday', so you write it down for future reference.
Gon
Oh, this sweet baby boy gets you everything. He doesn't care if he's out of cash afterwards. He'll get you as much as he can.
Killua
Killua will get you exactly what you want, when you want it, and where. He won't hesitate, even if it's something that's barely possible to obtain. He'll get it no matter what.
Kurapika
He'll get anything you want, as long as it's not the eyes of his clan. He doesn't want anything in return, but you get him something anyway.
Chrollo
He's gonna get what you want, with a Bible and cross as a bonus. In return, you give him a cross necklace. Priest gone wrong amirite?