1. A medical system comprising: an implantable medical device that that delivers a therapy to a patient, monitors a signal that varies as a function of patient activity, periodically determines an activity level of the patient based on the signal, and associates each of the determined activity levels with a therapy parameter set that was used by the implantable medical device to deliver the therapy to the patient when the activity level was determined; and an external programming device including a display that is configured to receive information identifying a plurality of therapy parameter sets and associated activity levels from the implantable medical device via telemetry, determine a value of an activity metric for each of a plurality of therapy parameter sets based on activity levels associated with the therapy parameter sets, and present a list of the plurality of therapy parameter sets and activity metric values associated with the therapy parameter sets via the display.