Maybe work up an arrangement? Have a 'loud' section and a 'soft' section. Get the players used to playing within a song and thinking about when they need to switch to the next section, even if the only difference is how hard they are strumming.
Have some players arpeggiate and others strum. Then have them switch.
Vary rhythms and feels. Play the 3 chord song in 4/4, 3/4, double time, half time, etc.
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