Lots of crazy options, and romantic ones as well. Flowers in Cancun are incredible, and reasonably priced. The Premier desk or the consegiere (spelling wrong) would be a great help. La Parilla and La Habichuela are two downtown restaurants that are great.
Be sure and let the staff know it is her birthday, they will have everyone around the pool sing Happy Birthday to her!!
if you are with a big group or you want to have more of a bithday celebration - pericos downtown is a great place to be entertained and the food is well above average (but most would agree the food is not as good as the two places bart mentioned) if you stay at the resort on her birthday night be sure and eat at the italian rest....like dave said - just let them know well in advance - they will look out for her very well
