Wanderings is a location tracking application and accompanying web site (at wanderin.gs) that shows a heatmap of where you've been. It is low effort; just keep the app running in the background on your phone and it will record where you are occasionally using GPS and other location methods. Then come to the website to see a heatmap of where you have been.
Wanderings is privacy friendly; we don't keep a copy of your location data on our own server. Your locations are only stored in iCloud (requiring your password to access) and cached in your local browser for display. The location tracker on the iPhone is designed to have very low impact on your battery; a few percent a day at most.