What did Harry bring back in time with him?
I think Harry should have brought everything he had in Gringotts, all the winn loto number and gambling information, all the history information he could get his hads on, and anything else he could to help him.
Author's response
About a million pounds in cash and gold, some secret agent toys, and the clothes on his back. Oh, and his wand.
You have to remember, he probably didn't have access to his Gringotts account, never paid attention to the lottery or the stocks, or anything like that.
I considered having Hawthorne giving Harry a paper with various stocks highlighted, but scrapped it. The fact that Hawthorne just HAPPENED to have a million pounds in cash and coin is questionable enough, I didn't want to have to deal with the can of worms that I'd open if Hawthorne knew exactly when Harry would end up, never mind the peculiarity of keeping a newspaper almost thirty years old to begin with.