Sometimes we are confronted with a chord that last several measures.  Repeating that chord for an extended period of time becomes monotonous and leaves the piece devoid of motion.  When confronted with such a situation, there are several, or possibly many strategies.  I'd like to discuss one of them here.  Here's one way of adding a chord or two to give that section some sense of motion.  You can simply add the next chord in the diatonic scale.  That may sound complex but it really only means, in a practical sense, playing the second degree as a minor seventh and then simply returning to the original chord.  For example, I was playing, "Dream" by Johnny Mercer.  The first chord (in the key of Bb) is of course, Bb. that chord is held for two measures, which is four beats.  I think that sounds too static.  So, I play two beats of Bb, two beats of Cm7, then go back to Bb. You can use this additional chord in the left hand while still playing a single note melody but you can harmonize the melody with these chord tones under the melody.  In our example, that would mean that while holding the Bb melody note, you can play, in ascending order, C-Eb-G under the Bb on beats 3 and 4 and then go back to the original Bb in the next measure.  Try it out.  I also do this on "Sentimental Journey" in the key of C. That means the progression  is C for two beats, Dm7 for two  beats, then back to C.  

@richardgoodman

Great one, Richard. Utilizing the diatonic chords to continue the momentum of such a musical passage can be very effective. One simple movement as the one you mentioned can make all the difference.

This tip is so "in tune" with a post that I created. It illustrates that this technique can be used even more extensively by including the iii and IV of the diatonic scale. This can be effective when a melody note lasts for four beats or more. I have referred to this technique as the "The Diatonic Surf" 😎 Here it is:

https://cocktailpianotips.com/cocktail-piano-intros-the-diatonic-surf/

Thanks, Richard, for bringing this up! 🍻