Food and treats
Since each dog has different feeding requirements, it is best to hold off on purchasing the food until you have adopted your Lab. We recommend feeding high-quality dry dog food available at pet supply stores. What is a high-quality food? See The Whole Dog Journal's annual reviews of dog food.
Most Labs adore treats and will eat anything and everything you give them-- but they shouldn't! Labs gain weight easily and treats are generally high in calories. 
Purchase high-quality treats (read the ingredients) and hand them out sparingly. Dogs respond to the frequency of treats rather than the size, so purchase the smallest size treats.
As an alternative to dog biscuits, see if your dog will enjoy vegetables (not potatoes). Carrots are a great treat.
We do not recommend rawhide. It is made by processing animal hides with strong chemicals. Rawhide sometimes brings out protective behavior in dogs. Rawhide can be ingested in large pieces, causing choking and major internal problems.
Similarly, your dog may enjoy a sterilized marrow bone when you are home, but do not leave him alone with it because the bone may splinter.