When you get to the park, hold down the lower LEFT button to "reset" the 305 and start a new ride. Then ride your lap(s) at the park. You can hit the lap button each time if you want to ride more than once, or not. Then reset again before the ride home.

When you get home and download your data, you'll have three separate rides listed. The first and third will be your rides to and from the park. The middle ride will be your lap(s) at the park. Make a course out of that one.

Any time you hold the reset button, it does not erase data and throw it away. It just starts a new ride, zeroing all your time and mileage stats to start the next ride.