There have been several of these types of bags being pinned. Totally clean and simple and the sort of thing you can get done even if you are pressed for time.
All you have to do is punch top of your paper bag with a decorative edge punch, then fold the top over. In this version, you can see the string wraps all the way around the bag and actually ties the tag in place. I used the sentiment from AppleDapples Collection: Filled with Cheer Emma Stamp Set, stamped it in black and embellished with some stick-on pearls.
You don't have to do the string this way, if you don't want to. I have seen some that are using mini clothes pins, shaped paper clips, etc., to attach the tag at the top of the fold. Just as simple, and still cute!