I said the car.
My theory is, that Casey would use an OTC drug (benadryl, nyquil, whatever) on Caylee to get her to sleep. (Especially when she was not at her parent's home.) I know that no one has mentioned this, but I believe that when Caylee was not with her grandparents, she was with "zanny the nanny." "Zanny the Nanny" being the trunk area of Casey's car.
Casey would give darling Caylee a dose of a medication, and she would fall asleep just in time for Casey's party time would start. Due to the use of these medications, Caylee had a hard time sleeping on her own (something witnessed and testified too, IIRC).
So, when the OTC meds didnt work anymore. Casey needed a new plan. In comes the chlorophyll. Once the research was done, Casey first tried this on the dogs in the home. When Casey felt confident it would work, she dosed Caylee with it. Darling Caylee fell asleep, and Casey went on with her night and extended morning in bed with the B/F. Possibly, leaving Caylee in the car this whole time. Maybe too much chloroform was used, or Caylee woke up in the car and could not stand the June temps in the hot car.
Caylee was probably used to Mommy's rules to wait for Mommy to get her. So she probably just played, maybe sang, and kicked her feet in tune with the music in her head. Eventually, little Caylee's body couldn't handle the heat anymore and that was it. I believe this is why the neighbor saw Casey BACKING the car into the garage the next day.
While the act alone which caused her death would be considered more likely the "accident" that did happen. Which I actually believe would be more like neglect, and direct harm to a child just so "Mom" & "Dad" can't run her life.
This is just my theory of course.