Go for a walk in the rain without umbrella's for an incredibly intimate and romantic experience. Pause under a tree, wipe the rain drops from her lips, lead down and kiss her, then drop to one knee and pull out the ring.
Turn off the lights and cuddle up during a thunderstorm - propose as you see each other through the lightning flashes.
Splash in the puddles and act like kids again. Bend down and tell her that you found something in one of the puddles. When you come up, stay on one knee and present her with the ring and your proposal.