I'd time warp back to when I was 19 and started smoking.
And then I wouldn't smoke.
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And for the World:
Easy peasy. Just make religion an unknown concept.
No religions, no divides, mankind as one.
Yeah, right, but I can dream can't I?
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PS: Oops, I kinda misread this, I was influenced by previous answers, so here goes:
A round the World trip with unlimited spending money.
And a two seater Morgan Roadster circa 74 to do part of the trip with