I think Donnie, himself, traveled back in time. I think I figured out how to do it. After all the bad stuff happened, he decided it would be better if he did die. He was laughing at the end because he figured out how to do it. That's why the bunny guy was crying at the end because, for some reason, he feels bad about something a.k.a hitting the girl. That's why the mother feels like she knows the girl when she rides by on the bike at the end. Because it all happened and Donnie figured out how to go back.