This is shameless self-promotion, but if you have an iOS device the Hopper app has a feature that lets you watch flight prices for flexible dates/length of stay

This is shameless self-promotion, but if you have an iOS device the Hopper app has a feature that lets you watch flight prices for flexible dates/length of stays/destinations and will send you a notification when there's a deal for any of those things. Disclaimer: I work for Hopper