I have always doubted the report from the lawn watering man and his wife, where they said that Elizabeth rode her bike to their street every night and that she visited with people on the block.
If the girls asked for directions to the park, then I think that someone told them that something special was going on at the park. I really doubt that they all-of-a-sudden decided to go there on their own, especially if they didn't know how to get there.
They were headed away from the house and away from Fareway store when they were seen on video. The Fareway store appears to be about a block East of the house. Maybe they turned right/South on River Forest Road, went one block, rode East to the Fareway store and then headed South on Elmer.
I can see a way both could be true. Put your mind inside the mind of a child. Elizabeth may have gone down to visit friends near Mr. C's house before. But if she had never been beyond there, she may not have even realized there was a like just beyond their houses. Or, if she knew there was a lake, she probably didn't know it was "Meyers Lake."
My daughter would say, "let's go to the park." But if I asked her to tell me the name of the park, there is no way she could, and she is almost 10. So when someone said something exciting is happening at Meyers Lake, Elizabeth probably just didn't realize she had been there before.