Welcome aboard, Melanie! Congratulations on your first cruise with Disney!
Check out
Disney Cruise Line's Port Adventures website. There you will be able to choose each port and view the available adventures there. With a four year old, be sure to also click the "Family Adventures" box on the side to ensure that most of the adventures shown will be appropriate for his age. (Some of the family adventures do start at 5 and up, but many are all ages, so just be sure to check.)
Since you leave May 10, you should already be in your window for online check-in and able to choose activities each day to add to your itinerary, including Port Adventures and Adult Exclusive dining. Once you log into your reservation, click on "My Cruise Activities" where you can add your extra adventures and dining. I highly recommend a romantic dinner for your husband and yourself in
Palo. Though there is an additional fee, I promise it is well worth it!
Finally, don't forget to pack pirate gear to wear on
Pirate Night which should be on your 6th night (Wednesday). Be sure to pack formal and semi-formal wear for those two evenings on the ship. The remainder of nights onboard will be Cruise Casual.
I hope that information helps, Melanie! Enjoy your Magical cruise, and let me know if I can help with anything else before you set sail!