I'm in the process of learning a waltz called "Aanisen aalot" (the Waves of Lake Onega) by George de Godzinsky.

What makes this an accomplishment for me is that it uses chords on the treble (keyboard) side of my accordion as well as on the bass (button) side. This is the first piece I've ever played using chords on the treble side. I'm getting pretty good at it, and I really like it. It was a bit intimidating at first, but now it's actually quite fun.