This is a good k'nex grenade, there is a low piece count, the pin stops it from exploding all the time, and it makes a nice explosion.
The idea is simple so sorry if this has already been made.
EDIT: I have made a better grenade at http://www.instructables.com/id/Knex-War-Granede/
Step 1: PIECES
The pieces are:
2 white circle connecters
16 red corner connecters
8 orange straight connecters
2 grey one slot connecters
1 yellow rod
1 rubber band
Step 2: TOP
Wedge 8 of the red corner connecters into on of the white circle connector.
Step 3: BOTTOM
Repete step 2.
Step 4: SIDES
Wedge the orange straight connecters into the bottom.
Step 5: ATTACH TOP, SIDES, AND BOTTOM
Wedge the top into the orange straight connecters.
Step 6: ADD RUBBER BAND
Put the rubber band around the orange straight connecters.
Step 7: PIN
Make it like the picture.
Step 8: ADD PIN
Put the pin through the white circle connecters and clip the end on.
Step 9: USING
Remove pin then throw.