Whatever you do: not games! Ok for a bit but believe me, spend a week of holiday playing one and you feel like you just cut a week out of your life at the end!
An hour or two here and there, but dont go nuts!
Write songs, learn new techniques, if you dont have a band form one, get on the train to wherever, sports, random pseudo-dangerous things (bungie jumping, hang gliding, what have you), I dont know - even belfast is pretty big, there'll be plenty to do if you look.