If you're looking for an action packed location, Isla Mujeres isn't your place.  As you suggest, 3 days in Cancun and 3 on Isla might work.  Most isla bars are closed by midnight.

 

What is you daily budget for food, beverage, and accomodations on Isla?  Also, when are you going, as accomodations increase about 35% around middle December, and stay high through February, so keep that in mind.  I'll be on Isla Dec 4-11.  If you happen to be there then, I'd be more than happy to show you around.  I'll have a golf cart.

 

Once I have your budet..............floor and ceiling..........I'll give you suggestions on accomodations.  If you decide to pull the trigger on Isla, then I'll give you suggest restaurants and watering holes.

 

Good luck and enjoy wherever you decide to go.