Chocolate, Raisins, Grapes, Onions, Macademia Nuts and Xylitol are ones that spring to mind....
Hershey's is milk chocolate, isn't it?? Marty should be fine - may well have an upset tummy, but do keep an eye on him, some dogs react more than others (more serious side effects, normally only associated with larger amounts, include tremours and convulsions...). Honey had a small bar of good dark chocolate when she was a pup, and suffered no ill effects at all!
I think grapes can cause kidney damage...