Very sweet of you to think of them! I do agree that one isn't necessary, but since gifts are actually supposed to be given from your heart rather than b/c you're "supposed to", if you still feel like giving them a token to celebrate the event, then go right ahead!
One budget option is to purchase for them a Rainforest Pass from the spa. You could spend more and do a length of stay pass (I think $99 for a couple), or just a day pass for, I think, $16/per person, per day. If you think they won't use it or have time for it, you could just order a bottle of champagne for their room. The prices are below, just don't forget that they tack on a 15% tip to this amount:
Champagne and Sparkling Wines
Item 1186: Iron Horse Brut Fairy Tale Cuvée Champagne, California, $52
Item 1187: Korbel Brut N.V. Champagne, California, $29
Item 1189: Möet & Chandon WHITE STAR Champagne, France, $79
Item 1190: Martini & Rossi Asti Spumante, Italy, $28
Item 2031: Möet & Chandon Cuvee Dom Perignon, France, $225