A good thing about the Grandeur is that it doesn't have as many revenue centers (places to spend more money) as larger, newer ships. There are no extra-fee restaurants for example.
Bar tabs are often the largest item for passengers because sodas, bottled water, and of course alcohol, all carry a fee.
If you buy excursions you will pay more through the cruise line than by doing it on your own. The usual caveat is "if you buy from the cruise line the ship will wait for you if you are late getting back". This is true. But with the places you are going to you will be quite good to do your own tours and save some money. This is especially true for Cozumel where private tours are plentiful. Or you simply might prefer taking a taxi to one of the beaches.
I am not big on Costa Maya since it is a created port with a faux Mexican village at the pier where things are rather expensive. Head south to the small town (it is just a mile or two) and things are more rustic.
Key West? I don't see a reason to do a tour there. It is the perfect walking-around port, or by an all-day pass on the trolley where you can hop on or off the trollies all day. They have a schedule so you won't be stranded. I prefer to just hang out in the downtown area.
Just keep the time in mind and stay with whatever time the ship is on, which may or may not be local time. They'll let you know what time to use.
We were on the Grandeur for an 8-night cruise to Canada and New England last summer. It was a great cruise and I really enjoyed the ship.