Things i have found handy on my wanderings are,
Those zip lock bags that have a one way vent that allows you to compress pack/stick your dirty laundry away and not pong up the rest of the ruck sack.
Waterproof case for digital camera , not an expensive one that goes to fathoms deep more like one for 5 mtrs deep , great for snorkeling, if your cousin is into scuba most dive shops will have the case for deep dive photos.
Not everyones fav but a good pair of sturdy "Teva" sandles , great for treking on forgiving ground, though not great in dem der hills!!
A silk sleeping bag liner is a must as some places have a relaxed attitude towards washing linen between guests.
Speakers for the MP3 player.
One of those travel clothes lines , the ones that don't need pegs with 2 suction thingys at end.
A few tacky keyrings , stickers , badges etc with random Irish slogans , flags etc great for giving to people in guest houses/hostels that have done you a really good turn.
Last but by no means least regular weather updates from home cos no matter where your cuz is , no matter how homesick they may or may not feel, when they hear that its p*s*ing it down in good old Hibernia they will smile
Hope I've given you some food for thought
Movax