Here's something I did once.
You know the old 1-2-3-4 chromatic exercise you do on all of the strings when you learn how to play?
You know the the one when you sequence it in groups of 4 playing 16th notes?
One day I decided to try sequencing it in groups of 3 playing triplets,
sequencing it in groups of 5 playing quintuplets,
sequencing it in groups of 6 playing sextuplets,
sequencing it in groups of 7 playing septuplets,
sequencing it in groups of 8 playing 32nds
and sequencing it in groups of 9 playing nonuplets.
Each of those challenges you in a different way. When playing sextuplets, while rhythmically much simpler than playing quintuplets, the 1st note in each group is fretted with the same finger that played the last note on the previous group, but on an adjacent string - finger rolls aplenty, and it's a bitch. When playing septuplets and upwards you'll have to do a lot of string skipping.