A gift card can be created in any reasonable amount. If purchased online, it will generate a code and send a virtual gift card image.  Make sure they bring the gift card (code) with them to their appointment, to present at check out.  It can then be used in any way that they choose.  That includes massages, enhancements, gratuity, and the gift shop.  There are many factors to consider when deciding an amount, a few suggestions are:

For a one-time massage experience:  $100, $150, $200, $250   Add $20 if anticipating any upgrades or enahncements

For massage packages, the gift that keeps on giving: $500, $775, $1000, $1600, $1900

The gift card does not have to cover an entire purchase, but the client coming in will be responsible for any remaining amount.