The graphics in Zapper are simple and attractive, without being overly mundane or boring. There's enough glitz and humor to keep you entertained. The graphics also do an amazing job being very involved in the actual gameplay. You'll have to navigate logs, spinning gears, and outrun some cute little cacti. There is also very good use of lighting throughout the game. Very often you'll be navigating areas lit only by fireflies or the antenna on your head.
As I try to think back to the audio side of the experience, I can't really think of anything bad to say. Funky describes both the effects and music very well. Zapper comes off as being something of a smartass visually, and I think the sound relates that irreverence very well. The stages themselves don't really tend to have too much music in them, but the music that is there is again, funky and hip. Either that, or it's funny. A remix of a certain classic western song while you are fighting in a desert corral comes to mind.