I wish I'd have had the confidence and knowlege I have now when I was young enough to make use ot it, or that my body was still capable of that amount of energy.
I think I'd do property development, I just got my DIY skills up to a reasonable level when my body started packing up, frozen shoulders mean I can't decorate and use tools properly as my arms go stupid when I try and hold them at or above shoulder height. My back will no longer allow me to lift things nor will my knees, nor will my knees allow me to kneel or bend down anymore.
I could do without many things that come with youth, like a functioning libido, menstruation etc, but a fully working skeleton would be great.
What would you do if you had the energy of youth with the wisdom of being older?