Everyone loves sunflowers so here's an easy one on a card in watercolors.
Here's how it looks at each stage in the process. A bit of ink and drawing at the end is what makes this design pop. I just filled in around the flower with an assortment of leaf shapes and used my pen to bring them out.
Here's the corner of the envelope that I painted for the sunflower card. As you can see I'm still experimenting with a bunch of different designs to come up with a center that I really like. Experiment with it and find one that works best for you.
Here's an envelope for my next card. Same basic idea, different colors and petal shapes. I call this one my rose and it fills the lower left hand corner of an envelope.