Fused dead reckoning can work, especially if you mix camera and IMU (or more than one IMU!).

Surprised nobodies mentioned celestial sensors.

Celestial navigation by imaging the sky, and combining that data with a clock and compass.

Cruise missiles have an optical terrain-following system, that would be a viable approach for drones.

Another technique not mentioned yet: compss augmented with local magnetic field features.

Inertial navigation and maps.

See the paper, but TLDR: stereo video depth sensing combined with fading previous measurements (so if you detected an obstacle at 1 o'clock, 2 meters out 1 second ago, it is now very likely at 2 o'clock, 1.3 meters out. This way you can limit yourself to just detecting things 2 meters out)