I can't speak for 2017, but during my plebe summer they allowed candy ... so there's probably no need to smuggle it in a care package. In fact, candy was the most common thing sent to us behind granola bars.
I haven't heard of anyone smuggling a cellphone (which would be a big no-no), but I know a couple of people whose detailer's overlooked the caffeine products in their care packages (think 5 hour energy or Mio Energy). I wouldn't be suprised if people got away with smuggling a little caffeine contraband in a pringles tube or bandaid box.