Bump Cube/Mirror Blocks
The bump cube is a shape-changing variant of the standard 3x3x3 cube. The layers are of different heights and widths and the puzzle becomes bumpy when scrambled.
The bump cube is a reasonable challenge. Some of the pieces are similar and, when the pieces are rotated, it's hard to spot exactly where they go. Layer-by-layer is my favoured approach for this and the odder of the shape changing cubes. It makes it easier to identify the pieces you are working with.
Although these look very shiny on the web sites, mine is just a cheap cube with iffy stickers. Don't get fooled into thinking that these things are made of solid gold or silver.