Final Fantasy XIV Database - Ffxivpro.com

It's kind of you to consider such things that would normally fly over most people's heads even if it is only in game. My suggestion would be decent food, fireworks, flowers, useful consumables like ammo/ninja tools for applicable jobs the bride and groom play. Just don't go too crazy in spending though, the key is showing your support more so than to one-up or impress others.

Ninja edit: Furniture is also a great idea, even though they'll technically have to share it.