Some ideas that popped up in my mind:
He goes back in time, but his time machine gets stolen and is going to be used for great evil, he has to stop them (bit cheesy).
He goes forward in time and sees how crappy/beautiful the world of the future is.
- If the world is beautiful: He wants to stay and tries to build up a new life but he soon finds out the world isn't that beautiful and he realises he belongs in his own time.
- If the world is crappy: He learns about the world of the future and after some problems he returns to his own time and tries to change the world of today, for a better world of tomorrow (lol that sounds like a miss world speech).
^I think at least one of these is an actual movie, not sure though.
He goes back in time and discovers a some dirty secrets about his parents/family, which will lead to a new adventure.
This is my favourite idea:
He goes back in time to once or more to one or more historic events. He then plays an important role in that event and ends up being on a famous picture/painting/etc we know today.
E.g. He goes back to the time of Jezus and plays an important role in whatever Jezus did exactly (I'm not religious) and ends up on that painting of the last supper.