my daughter used to wake up really early too- every summer since birth. I found it was just her biological rhythms. Nothing I tried worked, but here is what I tried, maybe it will work for you!
I blackened her windows thinking the early morning sun was waking her.
I put her to bed an hour later
I put her to bed an hour earlier
I cut down on her naps
finally I gave up and since my husband and I are both teachers and off in the summer (the one time we had to sleep in LOL)
we swapped days of waking up early and would take her to a local bakery for breakfast to keep the house quiet for the other one. They got to know us very well!!!
Now that she is a bit older (4 years) we bought her a special princess clock that puts stars on the ceiling and plays music from cinderella. It has big digital numbers. WE told her that she could not get out of bed until the first number says 6.
last summer we put a timer on her bedside light so that it would turn on at 6am. It was cheap-under 5$ and that worked, except that sometimes I think the light turning on woke her up. Anyways, we told her that she couldn't get out of bed until the light turned on. She was 2-3 then.