I've always thought that Casey drugged Caylee and left her in a house somewhere (maybe Lee's?). If she was leaving her in the car on a regular basis, due to the heat, Caylee would have died much sooner.
If the place (house) she left her wasn't available (people at home), and she wanted to go out, those would perhaps have been times she wanted Cindy to babysit, but Cindy didn't, and Casey couldn't go out.
I also think there were times when Casey was out with one bf or set of friends, and telling another she couldn't go out with him because she had to stay home with Caylee. We know she made up stories when she didn't want to go someplace with someone (Amy/Jacksonville/George had a stroke).
What convinces me that she drugged Caylee and left her somewhere is the night she told Ricardo that Cindy had called and made her bring Caylee home during the night. Casey, Caylee, and Ricardo went to sleep at Ricardo's. Caylee was not there when Ricardo and Casey woke up. Cindy said it didn't happen. Thinking logically of where Caylee would be, drugged somewhere in a house is the only feasible conclusion I can come to.