Here is a list of backer items in a blog comment.
With the stuffed animal + math trophy those are likely the 16 backer items.
Some of those were incorporated into puzzle chains but most are red herrings (with or without Easter egg use).
Other red herring objects are most of the stuff from the highway (e.g. Atari cartridges, plastic rings) and there were also red herring bags in the game 
(OK, the latter can’t be picked up)