You will find information related to the plugins here: https://build.phonegap.com/plugins/core and here http://docs.phonegap.com/en/edge/cordova_plugins_pluginapis.md.html#Plugin%20APIs
Create a setting page where the user can change some settings of the app (EG: on iPhone if the status bar should be visible or not ) The settings must be persistent. Restarting the app should load the settings at initialization. Also try building with phonegap build where possible.
Try capturing some media (audio,video,image) and upload it on the server. Take an image from the image library and display it in a HTML element. Make that element persistent upon relaunching the app. Play some audio files from your phones when different events happen in the UI.
Display information from geolocation, accelerator and compas on a page. Make those auto-update by using the watch methods.
Try interacting with a public plugin made by someone else