Oftentimes the value of a gift is not simply the monetary value of the item, but the representation of time, thought, and effort that the giver devoted. Gift cards are certainly a very convenient method to give something of value, but can be cumbersome to keep track of and redeem for the giftee.

If you as the gift giver want to bestow an actual trip (including effort of finding appropriate itineraries, confirming requirements, ticketing, etc.) vs. just some credit on an airline, you may consider creating some mock tickets to present at the actual event. Some cardstock, a little creativity, and a printer should allow you to produce a ticket-like coupon that clearly conveys the gift without locking you into any particular airline or ticket restrictions.

Print up something oversized and fun (the goal is to symbolize a ticket, not serve as a realistic replacement) with wording like "FROM: (home airport) TO: Anywhere!" to give at the ceremony, which will be treated like the real thing. You can then follow up a few weeks later to get an idea of the couple's destination preferences and date restrictions and start the actual booking process.

This way, you will be within most airline's 330-day availability window, you can have the couple's input on the gift, and yet you still get to provide value as a gift giver above a simple gift card.