USMC recruits can receive only letters and pictures UNLESS they specifically ask you for something. These will be items approved by their DIs. I sent my daughter cough drops (this was a substitute for hard candy) and Power Bars. And do not frou-frou up the envelopes. Plain white is best.
Once she graduated from boot camp and finished her MCT (Marine Combat Training), she went on to her MOS (Military Occupation School) in Southern California, and then on to Okinawa. In Okinawa, she was able to have any personal items she wanted.