Here I suggest a couple of ways to make better use of exercises that are 'too easy'.
To demonstrate, we have a look at the C Major scale exercise mentioned above. These are the first two measures:
Suggestion 1
Play the exercise with triplet feel. Triple feel (or 'swing feel') is often applied in jazz and blues.
Suggestion 2
Play legato (with hammer-ons and pull-offs).
Combination of suggestion 2 and 3: play with triple feel and legato.
Suggestion 4
Repeat each note
Suggestion 5
After suggestion 4 this is quite obvious: play each note 3 times, or 4 times... etcetera.
Suggestion 6
Play the same notes, but in a different rythm. Here are two examples.
Suggestion 7
Apply suggestion 6, and make it a sequence of 16th notes as demonstrated below. Here it is applied to the second example of suggestion 6.
Suggestion 8
Play the exercise accelerando (with increasing speed) and rallentando (with decreasing speed).
Suggestion 9
Play the exercise crescendo (with increasing volume) and decrescendo (with decreasing volume).
Suggestion 10
Optimize the pressure with which you press the strings to the neck. Start playing the exercise by applying no pressure at all. Keep repeating the exercise, while gradually increasing the pressure. When the strings stop rattling against the frets and all notes sound clear, you have achieved optimum finger pressure: not too much, not too litttle. I would recommend to apply this to any exercise, and even to any piece you play. Once your muscles start to remember this and do it automatically, your playing may benefit from it enormously.
Some final suggestions
Try to combine some of the suggestions above. Use a metronome - except when you want to change speed intentionally of course. Record what you play, and listen carefully if there is room for improvement.