Bring the cowboy hat, belt buckle, and spurs. Boots optional if you can fit the spurs on sandales.

This is for school?

If so and if it is a particularly religious school then it would be business casual, if it is secular I would suggest wearing what is comfortable, Be aware that in the south they tend to crank up the AC in the summer so bring a sweater or long sleeve shirt you can pop on or off as needed if you are susceptible to being cold indoors during air conditioning season.

Screw that.  Dress to impress.

Thong?

Look at pictures on the school's website, of similar events at a similar time of year.  Most open houses aren't at all formal - whatever students would wear to class is fine, though usually attendees tend to dress more like what students would wear for a crit day with guest critics - so one step up from everyday student wear. 

An exception would be if it's a school that has a more formal dinner as part of its open house (in which case they would have told you that in the invite materials).  If you'll be going to a restaurant or to an alumni club or to a faculty member's house for dinner (all things that happen at certain schools) then you'd want to dress up slightly for that - though unless they tell you otherwise you could still expect nothing more formal than business casual.

this looks appropriate.

Is this UT? And you've already been accepted? Wear whatever you want. 

Wear that same old, baggy t-shirt with the stain on it that you got from that team-building exercise.