Why don't you give your vet a call? Tell about the plastic as well as the bread. If the vet isn't worried, he/she won't have you come in. There are foods fine for people that are dangerous for dogs, but I've never heard that bread is one of them. Our seven-month-old lab mix has eaten parts of plastic bags (is that her idea of recycling?) and hasn't had any ill effects. But it can't hurt to ask about it all. Your vet will tell you what to watch out for.
Seven-month-old pups are like big babies, and they'll try anything. In addition, Labs and Goldens are notorious for being "foodies." You'll want to put him in a safe place (safe for him and for your food!) for a while now when you can't keep an eye on him (i.e., showering - let him stay in the bathroom with you), and keep your counters clear of ALL foods, because you need to nip this sneaky-counter-surfing habit in the bud. You might even try spraying a bit of bitter apple on the counters and lower cabinets before you leave for work.
Mine hasn't tried the counter-surfing yet, but she raids trash baskets as if it were her solemn duty. If you were to come over to my house, you'd see trash baskets sitting on tables and beds during the day. She'll grow out of that eventually - and find something else to get into.
P.S. The American Kennel Club has put out a list of foods dangerous to dogs - it might be on their web site. Chocolate and grapes are on the list, plus raisins, macadamia nuts... there are more.