At each port, you can do the easy thing and purchase an excursion through the cruise ship. Although you are completely protected by doing this, you pay at least twice as much as booking it yourself and you negate the possibility of finding any gems off the beaten path.
My advice ... don't do the "touristy" things, unless there is just something you want to do. And, if you decide to do it, book it yourself instead of through the cruiseline to save you a ton of money.
On our trip to Hawaii we are going to do a zipline and attend a luau, which are "touristy" Hawaii things to do. But, we booked these things on our own.
We learned a very valuable lesson when we were in Bermuda. By not planning, sometimes you happen across the greatest experiences!
We were wanting to go to a lighthouse in Bermuda. We purchased the two-day bus passes, grabbed a map, and headed to the lighthouse on our own. We stopped at the designated spot and got off the bus. There were two roads ahead of us. We could see the lighthouse from where we were. One road went up a treacherous curvy hill that screamed "death", so we opted to take the other path. Little did we know that the other path would take us winding around for several hilly and winding miles. Luckily we had bottled water.
We did finally make it to the lighthouse, and upon leaving we noticed that the other path went straight down to our beginning spot. It probably would have taken us 10 minutes to walk it (that is if we didn't get killed by a car on the narrow road). But, the things we saw along the way on the long path were breathtaking and we would have missed them had we taken the easy path. Gives additional credence to "the road less travelled".
I am currently in the process of researching some out-of-the-way experiences for us in Hawaii. In Maui we have a zipline tour booked, and are renting a car to drive up the Hana Highway on the second day. But on the third day we are in Hilo on The Big Island. There's not much there. So, we are renting a car. I really, really, really want to swim in a pool at the bottom of a waterfall. And there are very few opportunities, if any, although I have found a possibility. But while researching I came across a website highlighting a 4-mile stretch called the Onomea Bay Scenic Drive. The authors took much time in laying out the specifics of this scenic tour, and have indicated a particular swimming hole at the end. I am hoping this ends up being an incredible experience.
Anyway, the purpose of this post is just to advise you to take a chance and dig deep in your research! You just might happen upon a gem that very few people knew about.