A fun little right-hand #guitar exercise I've been idling with

There's a Hole in the Bucket is just
D G A7 D
D G A7 D
in 3/4 time

I just play it over and over but differently every time. Pluck whole chords, arpeggios in various orders, playing in 6/8, playing in 4/4, different plucking or strumming patterns, etc.

You can get a LOT of different sounds out of this very simple chord progression.