I have to say something here. The information and facts are a little misleading.
The kind of tree sap matters. CSJ, this site, just one of many, lists “Evergreen of any kind (pine, cedar, redwood, etc.)” As unsafe.
Maple they find delicious.
As far as metal being bad for crabs it is a myth. Even iron oxide (rust). There is no evidence, credible article, or science. It got started some years ago because of pollution – rusted crab traps. The fact is nature loves the iron. Artificial reefs have their base made of iron. I can of course back this up if need be.
Crabs are attracted to anything shiny – or really anything reflective. They will at least investigate it. I used to keep colored rhinestones, moonstones, (glass I ain’t rich) and dice in my tank. Like any child they would be become bored with them after a while. Need to rotate items. A group member told me they enjoy the air bubbles in the aerator. Below is “Leela” , Amber M.’s playing in the bubbles.