Lockdowns imply the costly confinement of many healthy individuals for each infected person. Digital apps on smartphones can reduce this cost, but discussion has focused on contact-tracing apps. In addition to familiar security concerns, these can be exploited maliciously in order to disrupt activity or impose quarantine on rivals. This column proposes instead ‘Activity Apps’, which have modest data requirements and less disruptive potential, but can match individuals to activities in ways that may substantially reduce the costs of controlling Covid-19.