I just did a craft show last weekend and sold over $240. I made notecards that said "Life is better in flip flops" that I printed on my computer, and added flip flop stickers, insides were blank. 8/pack for $5.
Cardstock is white, blank inside. these are SUPER simple to make and you can add variations. I bought several different colors of envelopes, one pack of notecards would be Red envelopes, another with hot pink, another with bright orange. I think the bright colors drew attention and I had a line at my table almost all day!
I had these other varieties:
"Chillin & Grillin...hope you'll join us" (these had BBQ Brads attached and blank inside )
"A note from your Teacher" (added apple or crayon stickers)
"Just keeping in touch!" with cute flower stickers
"thank you" with variety of stickers or brads
I also had $2 & $3 greeting cards and only sold 6 total.