I struggled with ideas for this port too, but we had a really fun day!!!
We did the chachoban (ok I know I spelled that wrong) ruins, the ones that were fairly close. We had a nice drive thru a rural agricultural area where pineapples were grown. We saw children playing in the fields. People came out to wave at the bus, because it was still unusual to see traffic on that road (2003). The ruins themselves and the jungle they were in were both very interesting. We had a friendly and knowledgeable guide who spoke English very well.
Then we returned to town and swam in the pool, and walked on the beach. The beach is a bit rocky but very pretty, and although the little development is a bit disney like, the really try hard to show you a good time. They even employ friendly local people to play pool games with you. If water volleyball is not your game, they had free hammocks and lounge chairs on the beach for you to relax in, and a free cultural show with dancers. We had 3 teenagers with us, and all had a fun afternoon.
If you want a bit more rustic time, go to Majahal, a small town nearby. Warning (or a good thing, depening on your viewpoint), others with us reported that it is not at all touristy and their snorkelling trip was a bit hairy, but they enjoyed seeing a bit of real Mexico.
Good luck and have a good time in Costa Maya!!