To create this UI, I created a world space canvas and added buttons for each of the available hands. To each button, I added a box collider as a child object. A script attached to the box collider detects when a hand has collided with it. To detect a hand, I used the Leap libraries and then checked to see if the collision object is a Leap HandModel.
In this prototype UI, I am using large buttons for two reasons. First, reading small text in the Rift can be difficult, and second, while using the Leap allows me to see my hands, in my experience, it does not track finger motion well enough for detailed interactions to be effective. In several of the tests I ran, the user's hand was generally in the right place but the fingers more often than not were at different angles than the user's actual hand. The effect was that my users seemed to have the fine motor skills of a toddler - they could reach out and touch everything but they didn't have a lot of control. On the positive side, when the user has hands in the game, it appears to be very natural for users to try to touch items with their hands. Even when users don't have visible hands in the scene, you'll often see them reaching out to try to touch things. While I have the start button say "Touch to Start," once users know to use their hand to affect the scene they get it right away and don't need prompting or other instruction.
Leap Motion has just released a "Best Practices Guide" and I'll be looking at incorporating many of the ideas documented there in future prototypes.