I chose to walk to Fresco, it was a nice, refreshing (ok, I lied, it was BLOODY FREAKIN' HOT!) 25-minute walk from the Rental Car Center. There is a little outdoor shopping mall/plaza there and there's also a K-mart so it's perfect for bargain shopping. I have found that tickets Northern Europe - Latin America are quite expensive so often it's cheaper to purchase separate tickets via Miami and have a lot of safety margin hours in Miami. During such a layover it's perfect to go to this plaza to purchase Betty Crocker brownie mix, 87 different kinds of m&m:s, butterscotch morsels or whatever American food you enjoy, but can't get hold of at home. I can inform our Latin American readers that Spanish is widely spoken at the plaza.
Thank you everyone who contributed on this topic.