Anything that 2.4Ghz EM waves (microwaves) will pass through that has a different index of refraction than air can be used as as a lens or prism. A really common material for this is wax.
Also, you can reflect microwaves using metal surfaces. This is how a satellite dish works.
For a simplified model of your wifi router, you can think about the microwaves as a bright light bulb flashing on and off very fast. An antenna is mostly designed to record the signal ("pattern of flashes") rather than the distribution of microwaves in space. To get an idea of how the signal propagates you'd need to take intensity samples in the space around the signal and generate a volumetric map.