Carnival Glory - Shore Excursions?

Most excursions fall into 2 different categories- sightseeing and beach. You need to figure out what you're interested in.
You did not tell us what islands you will be visiting, so we can't give you specific info. Only general ideas.
When I go to an island that I've never been to before, I like to take an excursion that gives me an overall view of the place. If I go back, I can choose the places where I'd like to spend more time.
You need to read the descriptions of all the excursions.
I personally never take an excursion offered by the cruise ship. I wait till I'm in port. You will be bombarded with tour people. They will be considerably cheaper and better.

The best answer is what you want to do. There are many things to do and many different excursions.
You can search Carnival wed site for your cruise to see what is offered and prices.
It is true you can get off ship at port and get an excursion on your own but if you book through Carnival they stay in touch with excursion to help of you need it. If there is a problem Carnival will make sure you get back to ship. If you go out on your own and there is a problem and you don't get back in time the ship will leave without you. Not trying to scare you into booking with Carnival just want you to to make in informed decission. Hope you have fun and injoy your cruise whatever you do.