In the Castelia City Pokémon Centre, you will find a man who will give you gifts based on the amount of trades you have done. These can be trades on the GTS, over the Wireless Union room, over WiFi with people on your friend list and even the in-game trades. He counts the amount of Pokémon with IDs different to you and from that, determines a gift for you.
He only gives the gift once, but most of the items aren't perishable. However, once you reach the final gift, he will no longer give you any more items. Below is a list of the items you can obtain from this gentleman