Porridge for (part of) breakfast first (which I've finally trained myself to like, after being force fed it as a toddler and growing up hating it). It has steady energy release which lasts a long time.
Jelly babies, in a small screw topped container for in the cave. You can dispense them into your gob without the remainder getting wet. You can also "drip feed" the supply of energy, rather than having bigger snacks less frequently.
(I started using these for caving as a result of using them on fell runs, to maintain energy levels.)
Hasn't this topic been discussed before on here? There may be some useful info already available.