I've thought of this occasionally over the past days, but I really can't think of much. Solar panels and my Kindle? It'd be nice to get some reading done. Maybe some fishing gear and the means to make fire and chop up the catch etc.
I think I'd quite enjoy the peace and quiet really. How big's the island, is it worth bringing my mountain bike so I can make some sweet jumps?
"French explorer Samuel de Champlain's observation that the summits of the island's mountains were free of vegetation as seen from the sea led him to call the island L’Isle des Monts-déserts (meaning island of barren mountains)."
I'd much prefer a warm tropical island. So I'm going there.
Do you anticipate sharing these three or so months with anyone else on your choice of island or are you looking for a deserted desert island? If other people are taken out of the equation I'd still take the aforementioned items and a device for playing music and hope for an indigenous pizza oven.
I fear that my "hey, peace and quiet and stuff" would mean I was quickly very bored in reality, if it ever became reality. On the other hand, the time needed to get and prepare food might negate that. Should probably pack some antibiotics too.