I've already made most of my announcement for my husband. It's a video with 1 second snippets of our kids to the music 'Heartbeats' by Jose Gonzalez. Near the end there are a few screens with text saying how the kids think he's the best dad ever and now it's time to share him with someone else. After those screens I'll have shots of the positive pregnancy tests and kids holding signs (#1, #2 and #3 on my stomach) etc. that kind of thing anyway. I've had it made for months (!!! I have long cycles) and I'm so ready to use it. He's away a lot at festivals and I'm hoping I get to plant the video on his phone before he goes away for one and then set a reminder on his phone alerting him to the hidden video so he can watch it while he's away. I think the covertness of that will make him happy.
For trying for your first you could maybe do a similar video but shots of the two of you and then do a similar thing with text saying it's been an amazing life with just the two of you, but now it's time to share your love with another person. Also if you didn't have enough video snippets you could do photos in a slideshow instead?