If humans could see beyond the .00035% that the human eyes can see.

If humans could see beyond the visible light spectrum that our eyes are limited to (wavelengths of ~390-700nm), we would be able to perceive and observe many incredible new things:

Ultraviolet light - We could see fluorescence and glows from objects, animals, plants and minerals that absorb and re-emit UV light. Scorpions would glow under natural UV conditions.

Infrared light - We could see radiant heat emitted from warm-blooded animals, engines, electronics, etc. Human bodies would visibly glow bright against cooler backgrounds.

Microwaves - Microwave ovens would reveal their energy spilling out. Microwave and radio transmissions would become visible crisscrossing the skies.

Radio waves - Being able to see longer radio wavelengths would reveal the signals and EM radiation propagating all around us from gadgets, antennas, satellites, cell towers, etc.

X-Rays and Gamma Rays - We could observe high energy radiation and nuclear processes with the naked eye, but likely at the cost of being damaged by it.

Dark Matter - Potential interactions and observations of theorized dark matter particles if we could detect their frequencies.

Quantum Behavior - Possible observations of strange quantum effects taking place at tiny scales.

Overall we would see a much more energetic, vibrant and strange version of reality beyond the limited visible frequencies our eyes evolved to see. But there would likely be drawbacks and dangers to perceiving more of the full EM spectrum.