Not all areas have tour operators right near port to plan when getting off. There are several 3rd party companies online that offer excursions similar to the cruise lines and they tend to be cheaper, but may not include things like transportation from port to the activity so you have to add taxi fare and a bit of hassle.
Many people will rent a car or do things on their own but then you should do some research ahead of time of things to do in port. Simple things like hitting a beach, shopping, or finding a local restaurant can be done last minute but if you want to fit many activities in the 8 or so port hours planning is key to do it on your own.
Hiring your own tour guide is another option but will cost more while getting rid of the work.