Good game... I played it for a lot longer than I usually would play a random flash game.
Some suggestions or ideas:
1. Vary the background sound from level to level. By level 4, I was ready to turn the gun on myself just to get rid of the repeptitive background noise. The other sounds were great, though.
2. The levels felt like they dragged on just a little too long. Perhaps shorter levels that increase in difficulty gradually might work better?
3. The varied enemies are great, and the fact that they all behave differently is perfect. The "Ghost" box turning into a ghost and then coming back is genius. The cupcakes, however, weren't really hard to nail at all; I consistently got every single one and I'm not that good.
4. The bonus for the cupcakes wasn't much of a bonus. Maybe that's intentional because they're so easy to hit...
5. I'd like to see the background change with the levels, too. Maybe make the background at the higher levels so it's a little harder to see what's coming? Or a cupcake background? Don't make it too harder; just enough to screw with people a little.
Just some thoughts. Take 'em as you will.