When my stepkids were that age we involved them in all aspects of cruising (teaching opportunities!). They helped man the helm and were shown basic navigation techniques and read the chart (slightly older then). They helped plan and cook our meals and clean and maintain the boat. Lots of fishing!
I also had bags of simple arts and crafts projects (making pot holders, simple knitting, etc.) We dried crustacians in the sun and mounted starfish, crab legs and sea shells in our pilot house. And made gifts for family and friends back home.
I've also taped my reading books and my little ones could put their headphones on and follow along with the pictures in the book.
I purchased lots of disposable cameras and let them take whatever pics they wanted and helped them make scrapbooks after we got pictures developed (some very strange shots, I might add).
Hope your board games are the magnetic kind but keep them away from the compass
