Oh, I decided this time to answer my own question.

If I could, I'd learn:
- to play piano and guitar, and maybe sax or harpsicord
- to sing opera
- complete fluency in hebrew, yiddish, russian, italian, japanese, farsi, arabic, latin and norwegian
- advanced accounting
- how to snowboard without falling on my ######
- how to tie my shoes "the right way" (and save myself the constant delays as people point tu "your shoes are untied!")
- AJAX
- C++
- how to fly a cessna and a commercial airliner
- studio lighting
- how to make an amazing napoleon and bakery-quality rugelach from scratch
- which people say they're my friends, and which ones really are
- the next month's lottery ticket numbers
- how to invest wisely (even without the lottery winnings)
- conversant familiarity with local and israeli politics (and then be able to have an opinion on them)
- how to read people
- the lyrics to every song I like