Up and down the overtone series.
With many beginning and intermediate players, they start playing a piece of music beginning on the wrong note, and continue on their merry way without even knowing that their entire performance is off.
As soon as possible, I try to teach students about the harmonic series. It seems that if they have even a basic knowledge of it, it helps a great deal with accuracy. The idea being that if they know what the “extra” notes are in the practical range there are in a better position to know what notes are “right” and what notes are “wrong.”
For advanced players, working on harmonics exercises has tremendous benefits. The extra embouchure and breath control required to go up and down the harmonics series translates to better control throughout the range of the instrument.
Here is a basic routine that I do on a semi-daily basis. Once I set pen to paper (or more accurately, mouse to computer) I hope to post other harmonics exercises in the future.