Solo, shadow fighting/boxing is going to be king.
Next I'd say that basic rhythm footwork and head movement drills, and drilling the heck out of combinations (so they become automatic and you don't have to "think" about how to move your body while throwing them) are going to be my next favorite solo drill.
After that I would say drilling defensive movements against imagined attacks (the better you can get with your visualization skills here the better) and countering skills would be next.
Beyond that you can add "Angular Attack"/"5 Ways of Attack variations to any of these drills for dealing with different types of defensive responses from your opponent, rhythm mix ups, or strategic/tactical considerations.
Of course if you have access to targets (heavy bag, double end bag, uppercut bag, etc...) you can utilize those for many of these as well.