You are going to love Playa Mujeres, you are staying at Excellence? it's the only one I know in PM.
Do the bike tour. The one in the early afternoon. Get there 15 minutes early. The one later in the day gets to be a lot of bugs as the sun starts to set. It's a fantastic bike ride, and they take you through the part of the resort with animals. We saw awesome parrots, crocs, and a ton of other stuff, it was really neat. Highly recommend.
It was the most amazing time in my entire life!