There really is a limited amount to do in Broadbeach when there are thunderstorms and your flight has been cancelled, so we went and had a massage. It really hit the spot and was cheaper than many places we are used to. The massage therapists were responsive and ensured they weren't being too hard or soft. If it's rainy, and you're looking for something to do, these guys have a nice range of services and are quite friendly.
Approximate cost: $40