If I could fly to the past with what I know now AND a load of wealth, I'd focus on adding languages - I'd speak English, French, Spanish, German, Portuguese, and Math.
If I were a money oriented person, I'd add Mandarin. If I were projected way back in time - Irish and Innu.
I love being in places where you hear people switching back and forth between languages. I'm still astonished at being along the Gabon/Cameroon border, on a narrow dirt road in a fantastic rainforest, and realizing the kids spoke French, English and a couple of more local languages. They seemed to think I probably spoke German and Danish because of who I was traveling with, so my pride was saved. But with only 2 fluent languages, I felt kind of stupid.
When I meet people with 7 or 8 languages, I'm jealous. There are things I can say to myself in French that don't have an English equivalent. They don't exactly translate. It's the same with every language.
For fishkeeping, German would be very useful.