If you are simply looking for another layer of comfort for your mattress surface, consider a mattress pad.
 
Every rental is unique and while the mattress you are renting is likely quite comfortable, you can never be TOO confortable on vacation.  A mattess pad offers extra protection in case of spills on the bed as well as an extra layer of comfort.  
 
If you are staying for multiple weeks, please make sure to select the item(s) for your first week, and the additional item(s) for your second at a 50% discount.