I got myself some Fabriano stationery a few years ago and it is really a joy to paint on. Once I paint these cards, I have some cool art that can easily be mailed out to friends. The cards are high quality watercolor paper. I have used watercolor paints on them many times. But in this case, with these cards, I focused mainly on acrylic paint and inks, though they are mixed media. The envelopes for the Fabriano cards are made of a different paper, but they are also paintable.
It is a great way to loosen up and try out some new color combos and try art ideas and tools. I was just playing. I feel less pressure doing these than when I am making a painting, say. I can mess one up and just throw it out. I like the sense of freedom.
If you don't feel like mailing them out, they're easy to tuck into your art journal too. You could make some notes on the back, regarding favorite color combos and just what you used on a given card.