Perhaps focus on a few core mechanics that are simple enough to implement, but are generally just fun for a player to perform.
Think of your choices of mechanics as a deck of cards. It is up to you to make a game with those cards (mechanics) that motivates the player to win and have a fun time trying. The same 52 cards gets you Go Fish as well as Texas Hold'em. The same mechanics of running left and jumping get you Mario as well as Sonic and if you just add shooting you get Megaman as well as Contra.
I hope that helps you visualize and breakdown one way to think through the process of making a simple game.