Whatever you do, don't put the ring in a drink, or food. It won't be too great when she chips her tooth/swallows it.

You didn't state where you were going on holiday, but if it's warm - why not take her to watch the sunset and do it then, or whilst looking at the stars. They're my only suggestions and they may not be brilliant, but I hope everything goes well for you! Good luck.

That ring is very pretty, by the way. I'm sure she'll love it.