No it is not overkill to book an excursion on every Island If that's what you want to do. When my wife and I first started cruising we booked excursions on every Island. We wanted to see the Islands and how the people lived and just see the sites. It was wonderful and we enjoyed every minute. Some excursions included some shopping. If you want to do excursions then by all means do them. I might add though if you book your excursions thru a private vendor (that's someone other than the cruise line) just know you are on your own. If that vendor does not get you back to the ship at departure time you may (will) be left and the expense of catching up to the ship at the next port is on you. If you book your excursion thru the cruise line they will wait for you to return. Private vendors are cheaper just leave enough time to get back.
DW and I will be on the Jewel in December. Enjoy your Cruise.