Use varied strumming techniques. Strum hard with a pick sometimes, switch to finger picking, then choose a mellow strum that softens the music. Do this within the scope of one song or several songs.
Find inner voices. These are notes on the middle strings of the guitar. Emphasize these from time to time by playing them louder.
Use bass runs. Don't be afraid to stop strumming and simply play the bass line that leads to the next chord. Audiences will welcome the change of approach.
Play implied chords. These are partial chords, often the top two notes on the high strings of the guitar. Use these when you don't have time to play the full chord but want to suggest the chord.