Some mobile games have "watch ad to get XXX" feature. The ad is mostly video, reward is mostly ingame currency or possibility to continue.
I'd like to make something like this in C2, but what to use? AdMob?
Develop games in your browser. Powerful, performant & highly capable.
Yes you should use the admob, but if for mobile must be paid plugin that you find in the store. Always for the same item only to touch on button and run the event you want. You need not be after watching the interstitial. If various items is a little more work because you have to make appropriate treatments to generate a ranom items or lake type. You could work with array for this.
plugin: https://www.scirra.com/store/construct2-plugins/construct2-cordova-plugins-1662