Heifer International (the organization that provides livestock to those in impoverished countries around the world) has a very interesting (and free!) tour of their modern and ultra-green building, which is right next to the Clinton Library!
In addition to being one of the most beautiful high schools in the country, Central was the home and site of the Little Rock Nine during the Civil Rights Era. There is a great (and free!) museum across that street that is absolutely worth a visit.