Is there a simple way to diffuse the light of a "super bright" 5mm LED?
A total internal reflection lens with no direct path from LED proper to subject,
or a Fresnel lens based "diffuser" will do what you want. 

These are available commercially at from about $0.50 from western suppliers and for much less for identical product from Chinese suppliers. See below.

TIR or Fresnel lenses are available which will convert almost anything to anything - including light from narrow angle through hole LEDs to very wide angle radiation.



Cheaper, easier, maybe good enough:

Wide angle through hole LEDs are often referred to as Top Hat or Straw Hat LEDs due to their shape. The die is close to the external face and the package is often blunt ended to obtain the required radiation tpattern. 

You can get substantially LESS directionality from a typical existing epoxy encapsulated through hole mount LED by simply cutting part of the top off so that the end is "squarish" and closer to the die. In this case, cut off probably all of the domed part and some more. Go down too far and you risk interfering with the LED proper - but you should be able to tell how much you can probably safely remove by examination. This approximates a "top hat" or "straw hat" LED. You can round the edges but this will probably not be necessary.  Practice on a cheap LED first :-).

Commercial "straw hat" wide-angle LED. 





Wide angle domed orange Fresnel add on lens for 5mm through hole LED.

Data sheet - brief

Pricing - in stock in 1's $1.04



In stock Digikey in 1's



Squarer Fresnel for 5mm surface mount LED  



Dimensions

Data sheet - poor