You could feed the end of the airline into a powerhead or circulation pump. Or feed it into the Venturi port of a power head (if it is equipped with one ). Or feed it into the input of a HOB or canister filter. The pump would do the mixing although I have "heard" that this is not good for the equipment. I used an HOB for 10 months with no ill effects.
You could try feeding into an air stone, however, I am not sure how effective this is on its own. If placed near a strong current it might help.
You could try using a ceramic diffuser but that requires a good deal of pressure. You will feel like you are waiting hours for the bubbles to appear.