Ever since I read Imperial Earth by Arthur C. Clarke back in the 70's I've been fascinated with Pentominos. It's a simple concept that leads to endless puzzling about how to fit these pieces into simple shapes. Pentominos are shapes you can make with 5 squares fitting full-edge to full-edge. Removing rotations, mirror images and reversals, there are 12 distinct shapes.
What's included are the 12 Pentomino shapes along with 4 rectangular solutions boxes. If that sounds simple, by all means please try to find as many solutions as you can!
The 12 shapes are 2-color and make seeing the outlines very clear as you assemble them. The boxes are single color and are 6x10, 5x12, 4x15 and 3x20 in size.
According to many sources, the 6x10 has 2339 solutions (head start solution is in the picture!), 5x12 has 1010. 4x15 has 368 and the 3x20 has only 2.
Of course you can set your goals to other shapes, but I think there's enough challenge here to last a lifetime!
Have fun!