Syncopation: I have found when right and left hands are moving back and forth, it helps to give more variety to a piece. If my arrangements are ever sounding the same I’ll try to add some different syncopation.
For example, if you are in 3/4 time and there are 3 quarter notes in the melody, add 2 dotted quarter notes to the left hand. Now, that is a very simple example but hopefully you get what I mean.
So, if your songs are in need of some variation, play around with syncopation. You might find a fun sound!