Oooh I like this a lot. Bouncing off the walls is just irresistible. It badly needs audio, tho.

Thats a neat base game idea, maybe you can build on that and add some more stuff. 

Some things that instantly come to my mind:

• add audio (even if its only music it would add much to the experience)
• add a depletable resource like gas/battery for the mower, so the palyer would have to plan the mow to be able to finish it
• add obstacles (stones, ponds,...) or stuff you should not run over (hamsters, cables, flowers,...)
• more eye-candy! (I like the fireworks, but during the level, the cut grass could also spread particles, the mower could leave tracks in the grass, the overrun hamsters could... well... you know... etc.)
• local two-player mode to compete in  who cuts more grass, or even add a battle-mode :D (ok, that would change the whole game...)