RunKeeper (free) makes excellent use of the smartwatch form. The app is designed to track your walks, runs and bike rides while providing detailed ongoing info about your progress.
Anytime you start a new activity, RunKeeper places a card on your watch that lets you view your current time, total miles traveled and miles per minute. You can pause or stop the activity by using on-screen buttons or by tapping a microphone icon and saying "pause" or "stop." When you're finished, RunKeeper gives you a summary card that shows all of your stats, including totals for the aforementioned measurements as well as the number of calories burned.
An optional $9.99/month subscription offers features like long-term statistics.