Have the cards printed on a nice high end paper and charge a little bit more, depending on your target market. Print out samples for them to view at the event so they will know this isn't your everyday holiday card that you get printed at Target or Wal-Greens. Plan ahead of time and work with a company that can give you a quick turnaround time for printing such as McKenna Pro, Mpix, Millers or WHCC. Your customers will likely want to send out their cards soon.
Other options are frames and ornaments. You can find some cute novelty items at places such as Neil Enterprises and you may be able to order in bulk. But double check shipping times to make sure you get these items in plenty of time prior to your event.
You could also create some printed holiday product guides and holiday gift guides
and pass them around to your customers at the ordering session. And lastly, when the customer comes to pick up their order or their free 5x7 you should utilize this interaction and customer touch point by up selling again. For example, include a little card with a promo for a family session. Make sure you include a time frame of with the promo ends and so they know it is a limited offer.