The TranzAlpine regularly makes the list of the "10 best train trips in the world" despite having a modest train. Greymouth is a small sleepy ex-mining town with some charm but faded glory (if there was ever any glory to fade). Christchurch is gorgeous, a great city for walking around in and making discoveries, but you really don't have much time. The Arts Cente is worth a visit, see http://www.artscentre.org.nz/