Hi Stephanie,
Did you get any good responses to your question? I just started working as an EI provider for OT and some of the sensory items I keep in my car include different tactile play items (beans, moonsand, foam, paint, brushes, different homemade stuff), a blanket for swinging, and if I had room I would probably keep a therapy ball in my car for some vestibular/proprioceptive input. I guess that is about the items I keep in my car as I try to do some more vestibular and proprioceptive activities with stuff within the child's home.