Masyu (and I still don't know how to properly pronounce that) is one of my favourite
Nikoli puzzles. I find it to be quite easy to do, though sometimes too easy... I've created an extension of this puzzle, tiling the plane with hexagons instead of squares.
Instruction: Find a simple closed loop that travels between adjacent hexagons (cells) via the centres of the hexagons. At each cell containing a white circle, the loop must go straight through the cell, and then make a turn in the very next cell on at least one side of the white cell. At each cell containing a black circle, the loop must make a turn at the cell, and then continue straight for 2 more cells in both directions. The loop can only make 120/240-degree turns, i.e. no sharp 60-degree turns.
Here is an example:

Labels: hexagonal masyu, instruction