This exercise for breaking out of a rut uses a the principle of "restriction as the basis of development". It's particularly useful because it uses (arguably) the most commonly used scale by rock, jazz, and blues guitarists. The trusted and time-tested pentatonic scale.
The notes which make up this scale appear across the fingerboard to provide five, easy to learn patterns which utilize comfortable 2 note per string patterns within comfortable reach. Despite how easy each of the five patterns are to learn and play with, frequently only one or two of these patterns are used, and moving these patterns into other positions is the most common means by which many players change key.
The following exercise serves to address this by expanding the familiar patterns and approaching pentatonic scales from a horizontal point of view rather than a position based (or vertical) point of view. This approach forces exploration which wouldn't normally be undertaken by sticking to familiar patterns.
I use this as a good 2 hour pentatonic exercise:
Part 1 (1st hour)
Spilt up an hour into 6 parts of 10 minutes. Pick a pentatonic scale (what a pentatonic scale is, and what notes constitute each pentatonic scale in any key can be found in a number of places). Play just on one string, on each string for 10 minutes, exploring all the notes available on that string in your chosen scale.
Part 2 (2nd hour)
Split an hour into 12 parts of 5 minutes. For 5 minutes each, play on each of the five pairs of adjacent strings (1&2, 2&3, 3&4, 4&5, 5&6). Then play for 5 minutes on each of the four groups of 3 adjacent strings, (1-2-3, 2-3-4, 3-4-5, 4-5-6). For the remaining 15 minutes, play on all six strings.
Getting some form of backing track like a 12 bar blues, or a simple chord progression is also helpful because this will give a tempo reference for exploring phrasing.