Your spouse, knowing the bride and being a really great person, comes up with something sweet and adds a cool blessing. It's your turn.
The obvious thing- at least if you are me- is to just be honest:
"Hey, I don't know either one of you, but that was a great wedding. Thanks for inviting us, and thanks especially for the fajitas and cake!"On the plus side it's honest. On the minus side it comes off rather cynically.
But if you're me then your creative side takes over; you riff and create a memory. After adding a little bit of a blessing, you gaze off into the painful, invented past, and you speak:
"I think my favorite memory of you two is the time we went zip lining. I fell and broke my leg. I waited and waited for you to come help but it was three days before anyone showed up; I almost died. Then I found out you two were so in love that you suddenly had to have a second honeymoon. I couldn't fault you for that. Y'all rock."P.S. This is all true. Welcome to my life.