Sunglasses are less an option than a necessity for most triathlons, to keep the wind out of your eyes on the ride and block the sun on the run. We're sure some folks have raced with Wayfarers and similarly ungainly shades bouncing on their head, but a pair with multiple lens options (for cloudy or lower light days) that are insanely light (since you'll be wearing them for two thirds of the race) are a sensible upgrade.
Oakley's Radarlock weigh less than 30 grams (about an ounce), and have the best lens swapping system we've seen: Fold back the left arm and flip the temple hinge to swap out any of the the Radarlock's eight lenses. The field of vision is excellent and the arms fit snugly without clamping, so they feel comfortable – even on hour six. [$220; oakley.com]