me and my friends once had an elaborate discussion about just this subject. It mostly involved the world being devoid of any other human inhabitants, and us destroying entire cities piece by my various means, most of wich at least partially involved explosives. there would aslo be alot of street racing using various stolen and very expensive cars. As well as the building of ridiculous devices such as linear accelerators and trebuchets. I would also skate all the places the man tells me i can't... like inside malls... i've always wanted to skate inside a mall... damned security guards and their concern for the well being of the malls "customers"
I'd also try very hard to beat wilt chamberlains record... although it would be nice if i could do so while ensuring each encounter was meaningful, mutual and based on a firm basis of love and respect.
I'd also probably walk around completely naked alot. There would be alot of fires popping up in unexpected places and breakable things falling from high places. I'd also try and reorganize world government into a much more workable system.
I would'nt mind exploring all of the US governments secret millitary instalations too... i'm sure there are many fun things waiting there.... And it would be fun to try my hand a piloting a harrier... ooooh... and massive paintball games encompassing entire cities involving thousands of people... that would rock too....
essentially... without fear of consequence i would act out all of my most primal and childish urges that do not harm others... although i'm sure in the process of doing so i would do heck of alot of harm to myself... .