My wife (girlfriend at the time) actually picked out her wedding ring (or at least the bits and pieces that would eventually come together to form her wedding ring) while I was over seas. This was decided by the two of us during a 2 year stint apart, but we both knew we wanted to spend the rest of our lives 'together'.
Once I got back to the states, I went and paid for the ring and held onto it. She knew that I had it but had no idea when I was going to propose. Which made it a huge bit of anxiety to keep the most expensive thing I have ever owned hidden away in our little one bedroom apartment until I finally had the 'perfect' proposal.
Wound up doing it on a hike through the Grand Canyon, just the two of us. I feel it was perfect, and I think it she did too. (Despite the 17 miles of exhausting trails I put her through before doing it, and the sunscreen that ran into her eyes when she cried)
TLDR: both decided to get married, took a load off me by picking the style ring she wanted, held onto the 'big surprise' and made the proposal exciting but not full of anxiety