Flies are far from simple. Read up on their biomechanics sometime. They have six degrees of freedom--forward, backward, up/down, side to side, and rotating around all three axes. They're fantastically sophisticated flying machines.

Depends on how sophisticated you want the avoidance to be!

On one end of the spectrum, you could have the Roomba-style drone that physically feels the obstacles. Contact on a bumper would instruct the drone to stop, avoiding a flight-ending collision. It would probably need to fly pretty slowly though.

On the other end of the spectrum, you could match the incredible FPV drone pilot Charpu [1] with onboard AI. Flying full speed through very narrow gaps with a long term goal in mind.

[1] https://www.youtube.com/watch?v=1MBW8zoZUR4

Flies fly in far less cluttered environments than drones do (relative to their size and manoeuvrability). Something like a wood pigeon would be a better example. Still, it's not something that should require an AGI. :P