When we visited previously we took an excursion to the top of Mount Teide and would recommend that to any first time visitor. This time we took the shuttle into town and walked around. I think that is probably a good option for a short visit but next time we will look to see other parts of the island.
Oh dread, this island, for the cruise visitor, can be a real nightmare. (I remember it years ago when it had a gentle splendor.) We had seven huge ships in port and it was a mess. We chose an excursion to reduce the amount of hassle getting over to Marigot. If you can get to Marigot or the beaches and then hike around on your own, that is the thing to do. Otherwise, it is an overcrowded cruise port with the usual tourist vendors.