Games need a challenge that can be varied in multiple ways across the game and that is either fun or rewarding. Try to come up with ways to add all that to your game. I guess you've gotten the fun bit down, but you could add in in-game currency that are awarded for mashing things and allow the player to buy new stuff... that'd add in the rewarding bit, and possibly some variety. The hard bit would be challenge. I guess the easiest way to do this would be to have time limits you need to mash the stuff in, maybe more than one object sometimes, and then introduce harder objects as you go along. Then the player needs to buy new mashers and weapons and stuff, and also use the correct one for the job. Like some things are easier to mash horizontally than vertically, and some are easier to smash with a baseball bat or hammer than mashing with a hydraulic press, and so on.