Way back (oh, let's say 25 years ago), a company called Infocom was the dominant player in the "text adventure" field. If you've ever played "Zork," you know all about their products. If you haven't, by the way, you should! Anyway, most of the Infocom games after the Zork series included something called "feelies," a term that referred to physical objects included in the game packaging. The game "Witness," for example, included a matchbook, a letter from a deceased character(!), the front page of a newspaper, a telegram, and so on. Although none on the items were required to complete the game, they did add a great deal to the overall experience.
It's entirely possible to do the same thing with Wherigo cartridges. In "Closing In on D.B. Cooper!," I've got three such feelies hidden in various zones. One zone has an Altoids tin with a message inside, another has Cooper's necktie, and the third has a white mailbox (a reference to "Zork I"). I could've done the same with my other cartridges, but since they're set in public parks, I elected not to do so. The downside to feelies, of course, is that they increase maintenance tasks and possible muggle interference.
Parting thought: "Feelies" are a fun way to enhance the Wherigo experience.