I’m in total agreement with those above who recommend practicing bouyancy control in the pool. I wish more instructors would include this in the OW pool sessions. That would go a long way toward having new divers ready to dive in the "real" world without generating zero-vis for their neighbors. Just a little simple practice at staying mid-water for a couple minutes in the calm, safe environment of the pool - without the stress of being judged on a bunch of other skills - helps build muscle memory as well as confidence, which will help with buoyancy later during the task loading of skills on those first few OW checkout dives.
I also strongly recommend this same drill for any diver who wants to get to a good level of bouyancy control. If you can do it in a 6 or 8 foot pool, you’ll certainly be able to do it at 15 or 50 feet in OW. I also like some of the more advanced buoyancy drills (upside down, sideways, etc).
With regard to "tricks" and "snacks" - how about drinking some Capri Sun, eating some hard pretzels or a "halloween" size Snickers (the really tiny ones - anything bigger can’t be chewed and swallowed in one breath, we’ve tried), or cook an MRE (the heater is water-activated, so it works). How about gaining some empathy for handicapped divers. Swim "blind" (w/o mask, eyes closed), remove 1 fin and try to swim straight 1-legged, or put both arms behind your back and notice all the things you can’t control (and figure out what you can).