Monetize your free games by displaying different types of ads. Using the Chartboost ad service is a great way to have other games/apps and advertiser pay you for presenting their content to your game players. This is a great way to monetize a free game.
Chartboost is an advertising network that provides both an integrated full page of other third party games/apps as well as a fully customizable interstitial ad. You can define the frame/border around the interstitial ad and the image used for the close icon on the Chartboost website when creating your ad campaign. Chartboost works the same way on both iOS and Android devices.
You will need to create a chartboost account (https://www.chartboost.com/), setup an app, and retrieve your signing keys to test this ad network on your device.