Portraits are the hardest to knit. Since the knitted stitch is not square, the knitted face may be too long or too squashed. However, I find this gives a different face expression, and recommend not to give up. Go ahead and try - see what happens.
Nature Scenes are the easiest. Here the picture does not need to be accurate.
You can knit with different colors for a different effect and just have fun.
Conceptual Art is based on drawings and doodles I made. Like with the other knittings - I love making them and hope you have your own ideas that you would like to knit too.
Mandalas/kaleidoscopes are amassing to make. By the time I reach one third of the way, my mind gives up as it can not make sense of the picture going sideways and then upside down. That makes me feel like the Mandala is actually working on me.