Probably late eighties/early nineties.
Assuming I couldn't take an almanac or best-selling book series to plagiarise (Harry Potter, 50 Shades of Gray) back with me, I'd still sell the family estates (combined worth: ~300-400k), move into a small condo, and put the rest of our money and existing stock into Google, MS and Amazon stock as soon as they're on the market. Periodically, I'd use dividends to buy property I know will be heavily developed in the near future. Bet big in Vegas on anything for which I know the outcome, like the Douglas-Tyson fight and Duke beating UNLV in the NCAA finals in 91 (?).
Hopefully, as a part of all this, I'd be young again, simply imbued with my current knowledge
