In my last post, I went over the benefits of regular scale practice (and the resulting mastery of all 24 scales). Here are some ways to practice scales (and make them just a bit more fun):

Direction: 
1. THE BASIC: 1-2 octaves (young beginners) or 4 octaves (intermediate and advanced) in parallel motion. Hands move in the same direction at all times.
2. CONTRARY MOTION: Both hands start on the same key (e.g. for C major, both hands on middle C) and move in opposite directions. The notes will not be the same but (for most scales) the fingering will be the same. Two octaves each direction.
3. RUSSIAN SPLITS (late intermediate): Two octaves parallel ascending (start low), two octaves contrary and back, two more octaves parallel ascending, two octaves parallel descending, two octaves contrary and back, and two more octaves descending (to get to starting position). 
4. TRIPLETS (advanced): LH plays a one-octave scale low in the keyboard. RH starts an octave above and plays triplets (3 notes per LH note) for four octaves. Continue this pattern until the hands return to the starting position (it will take ___ complete RH scales to get to this point). Then switch: RH plays a descending one octave  scale at the top of the piano, LH plays a descending four octave scale in triplets. 

Articulation and Dynamics:
5. All legato.
6. All staccato.
7. Legato on the way up, staccato on the way down (and reverse)
8. One hand legato, the other hand staccato. Try for the entire scale, or switch halfway. Or switch every octave!
9. All forte.
10. All piano.
11. Forte on the way up, piano on the way down (and reverse)
12. Crescendo on the way up, diminuendo on the way down (and reverse).
13. Crescendo to the halfway point of the ascent, then diminuendo to the top. Same on the way down (and then reverse).
14. One hand forte, the other hand piano. Try for the entire scale, or switch halfway. Or switch every octave!


Variations:
15. THIRDS: LH starts on the tonic, RH starts a 3rd above (median)
16. SIXTHS: LH starts on the 3rd note of the scale (median), RH starts a 6th above (tonic)
17. TENTHS: Like thirds, but with an octave in between
18. OCTAVES. Both hands playing octaves. 

Some helpful links:
http://www.true-piano-lessons.com/piano-scales.html
http://www.pianocareer.com/piano-technique/piano-scales-arpeggios-art-exercise/
http://kantsmusictuition.blogspot.com/2007/09/method-of-practising-scales.html

Not many students would call practicing scales fun (at least, until they become fluent in them). When I ask students why we practice scales, broken chords, cadences, etc. I usually get a blank stare, unhappy face, and a response that goes something like this: "Because... we have to?" 

Scales are an essential skill for every musician. They develop:

• A practical understanding of theory. In western classical music we have 24 keys: 12 major and 12 minor. When we learn a new piece of music, one of the first things we need to heed is the key signature. A strong understanding of the hierarchy of the notes in that key (tonic, subdominant, dominant, relative minor... as opposed to just "trying to remember the sharps and flats") will very much expedite the learning process. In addition, we can better recognize and memorize patterns and understand the relationships between various sections of the piece (e.g. relative and parallel keys).
• Easier performance of scales in music. So many pieces have scales and scale fragments built into them! Once we know our scales, it becomes so easy to identify and play the scales within the music. 
• Coordination between the hands. There is a variety of fingerings for scales, and there are also a number of different ways to practice the scales that will very much increase the independence and coordination of the hands and fingers. 
• Stronger and fast fingers: All fingers are used and none should stick out as being too loud or too soft. If you can successfully play a scale at a slow tempo, you're not done your job yet. Work (with metronome) on increasing your speed. Try a variety of articulations. Listen to your tone quality. It's limitless!
• Keyboard topography: The ability to find notes without looking at your hands is really essential. Scale practice develops a strong familiarity with  the keyboard layout.