I have to say that I really don't think she was planning to attend a rodeo in Huntsville. First, the idea that she was just using the prison as a landmark and not a destination was already unlikely before, and everything we've learned about Sherri lately has made that less likely, not more likely as some have suggested.
Here's why: The information content of an observation is determined by how unusual it is. If an observation is common, its information content is very low, and the opposite is true if it is uncommon. Now, a 13-year-old girl who loves horses must be about the most common thing ever. My 11-year-old daughter, for example, just loves horses. So do her friends. All of them. So did most of the girls in my class when I was 13. So when I learned that Sherri loved horses, it didn't mean much to me. Because that's what I already sort of assumed before.
But there is one very unusual fact that we have all recently learned about her: that she has been associating with older males who were involved in criminal activity. Neither my daughter, nor any of her friends, nor any other 13-year-old girl I know does that. And no, this is not about blaming, but about showing what information is important and what is not. When I heard that, I changed my certainty that she really was trying to visit an inmate at Ellis Prison from "quite likely" to "extremely likely" because it's such an unusual thing to learn. Needless to say, all other possibilities became accordingly less likely.