I think day trading and GPU mining are the safest bets, though they are still bets...

Day trading will teach you skills you can keep with you and apply to any financial investment and even if GPU mining goes straight to 0 at least you still have the hardware. In a worst case scenario you're stuck with hundreds/gpus of GPUs and over wired warehouse at least you'll be able to either sell the GPUs or lease out your GPU power for deep learning/ai.