I decided I wanted to start doing the same and carrying personal items in my car for whomever I ran across who was in need. Here is a list of items that can easily be purchased at the dollar store and shouldn't cost too much to help several people in need:
Can Opener
Knit gloves
Beanies
Deodorant
Toothbrush
Toothpaste
Floss
Body wipes
Hand sterilizer
Small first aid kit
Snack packs
Useable water bottle
Poncho
Socks
Comb
Local support information
Mints
Femine products for women
Change
Shampoo/conditioner
Chap Stick
Razors
Shaving gel
Q-tips
This site sells a lot a great of mini items on the cheap too.