Ahoy, Laura!
Ahhhh, the magical
Castaway Cay "Double Dip" cruise - my very favorite itinerary of all! How awesome to have TWO days to explore Disney Cruise Line's private island paradise!
The last time my family sailed on a double dip cruise, the Castaway Cay 5K was held on the first stop on the island. (It will only be held one of the two days that you visit.) I remember on one of our first cruises, seeing only a handful of runners head off the ship to participate in the not-quite-well-known-yet 5K run. On our most recent cruise, there had to be around 500 or more runners heading out for a fun casual run around the island! It's free to participate, and you'll get a cool Mickey medal! Not a runner? There's usually also an Island Power Walk available, which sounds like a lot of fun.
Since cruise activities are always subject to change, you'll want to make sure and check with Guest Services once onboard to confirm which day they'll have the 5K on the island. You can also find information about the run in your Personal Navigator - a daily schedule of activities that will be delivered to your stateroom each evening.
If you sign your son up for
The Wild Side Teen Adventure, and find that the 5K will be held the same day, it shouldn't be a problem to switch his reservation to the other day at Castaway Cay. You'll simply need to stop by the Port Adventures desk to make the change. We were on a completely sold-out cruise a few weeks ago, and there was still some availability for the teen activity. (I wish they'd let adults sign up - it sounds like a blast!)
I hope this helps with your planning. Have a magical day! - Diane W.