A good one for those that finish fast or look bored.
Draw a 3 x 3 square. Place a letter in diagonally as in the example below.
N _ _
_ N _
_ _ N
_ N _
_ _ N
and then complete with words...
N O T
A N D
T E N
A N D
T E N
If thats too easy make the grid 4x4, 5x5 and so on
Alternatively
Think of a 3 letter word and place it in the grid to make words
H _ _
_ E _
Alternatively
Think of a 3 letter word and place it in the grid to make words
H _ _
_ E _
_ _ Y