You can't get any simpler than one-layer cards. The cards today are super simple, yet really pretty (esp. in person). A line image was simply stamped and colored in quickly.
For the first card, I used Copic markers and a clear Spica glitter pen. The color intensity is amazing, and this photo does not do the card justice! If I wanted to add a little ribbon before mailing, it could be easily tied on the side. I will wait to choose a sentiment when I actuallly need the card ... I learned that lesson slowly and I have an entire box of "thank you" and "thinking of you" cards to prove it! LOL.
The second card was colored with retired SU! Metallic pencils. I am sad these are no longer for sale, but at least I have them to use when I need them. On this card I did stamp a sentiment because it was the sample card for the Senior Center Fundraiser we held a few months ago. Again, a little ribbon tied on the side would be nice, but we'll see, I kind of like it plain too.
I really like the difference you can achieve by changing the coloring medium (or would it be media?). Using the same image, you can get an entirely different feeling simply by changing your coloring tools.