The fact that the proposal is coming should not be a surprise. Neither should the look and style of the ring. Just shop for it together.
The actual moment of the proposal, if well researched and planned, can be a fun surprise (if your proposee enjoys that kind of surprise. And there’s no need to get fancy. Just ask. Not for permission to propose, but for permission to make it a surprise.)
But even that shouldn’t be too much of a surprise.
Also, make sure it’s a “dress your best” kind of date, so your proposee will feel good about that way they look.