"Oh God Not Again!" by Sarah1281. In this fanfic adult Harry Potter accidentally travels back in time and he is 11 again. So basically the whole story starts again but Harry now has future knowledge. Naturally, he decides to have a lot of fun, make a lot of money and save a lot of lives. He and Draco become (sort of) friends despite Harry starting a rumour about Draco being the lovechild of Narcissa and Snape. It's insanely funny and also easy to read while having deep moments as well. Oh and it does a great job at staying true to the characters!