I like this idea, but would try to extend the water theme. Put plastic fish inside with a gift card to Bass Pro Shops! A gift card to the pool store or local water park would be fun too. I'm all about making the gift card connect.
[Creative appreciation] flowers bookmarks, so that your book out of the flowers. Clever design is convenient to collect and appropriate use. Hearted DIYer may wish to use this design to produce exclusive bookmarks.