I have three Ti-Nspire CXII CAS graphing calculators for sale in excellent condition. This is the CXII CAS, which is a more updated version of either the CX or CXi. It also uses the CAS system, so it's capable of far more than the Ti-Nspire CXII (without CAS) or more common Ti-84 CE models. 

This is an excellent calculator for college bound engineers and serious math and science students. Might make a good graduation present for your senior.

Each calculator comes with cover, software for computer, and usb cord. Retail, they're about $175.