We have been to Messina many times, the last time was this past November (we will be there again this November on the Enchanted). It is a quaint town to walk around, a bit hilly in places. My DW's favorite part of town is just a few blocks north of the port. It is a very nice shopping district with many cafe's and restaurants.
Also, as you exit the cruise terminal you will find lots of taxi's if you want to take a excursion. We have hired taxi's several times to visit Taormina and Mt Etna.