Mass incarceration is destroying a generation of our citizens. Laws and sentencing guidelines created in the 80s and 90s were extreme and counter-productive. As President you cannot change federal law and sentencing guidelines. But you can clearly send a message by pardoning everyone held in federal prison who was convicted of a minor drug offense, has no history of violence and has a record of good behavior. If this is done in 2015 (rather than after the election) it will make this an issue during the campaigns and set a precedent for states and governors.
This won't fix the mass incarceration problems facing this country, but it will force the conversation.
Note: The president can also pardon people who have completed their sentence. Clearing a felony from their record.