I love the second too!
Some of the sets come with sentiments...
Then there are ones that refer to magical-have a magical day...you are magicial....dream big...Believe....
You can also just use any general sentiment like thinking of you. There is no rule that says those have to be flowers or whatever all the time
Little girls love horses/unicorns for everything.In fact, there is some cartoon with them as characters. They also love mermaids esp since the Disney movie.
Anyone who loves the sea will be good with a mermaid.
They pretty much fall into the sames groups as fairies, dragons, aliens, monsters etc IMHO.