Do something with the enemies to make them look better. After you are done with those, add different backgrounds or something similar :) recommand photoshop or illustrator for the best drawing effects.
You can also design more weapons and scatter then all over the place. Do something unique like a throwing axe or bow and hide them somewhere. My personal favorite is bow and arrows, since it's a long-ranged weapon, and unlike bullets, arrows can be reused. In addition, you can stab someone to death using an arrow(choking someone with the string or slicing his throat with the arrow is also possible). Sorry if that sounds brutal xD