The harmonica is one of the few instruments that can be played without hands. There have been many great harmonica players who only played with one hand, or without using hands at all. DeFord Bailey is one name that comes to mind. He was a harmonica player who always played with one hand, and never used any hand effects. DeFord contracted polio when he was three years old; a disease that was almost always fatal in the early 1900s. He was confined to bed for a year and he was only able to move his head and his arms. This is the time when he developed his unique, one handed playing style. He would lie in bed all day, listening to the sounds of dogs howling, hens cackling, and trains rumbling in the distance. DeFord would then emulate those sounds with his harmonica! DeFord Bailey eventually recovered from the illness, and became one of the greatest harmonica players of all time. He inspired countless people to play the harmonica, and also influenced many harmonica players of his days such as Sonny Terry.
Here are some rare recordings of Mr. Bailey:
Gwen Foster is another name that comes to mind. Foster generally played the harmonica in a rack, and used his hands to play the guitar. He had a very distinctive harmonica style. I'd say one of the most distinctive styles in the history of the instrument.
I hope that you can now see what is possible on a harmonica without using two hands. "Wah wah" sounds are good, and it is probably not possible to create them without using two hands. However, they are not everything. There are lots of things that can be done on a harmonica without using hands.