Diluted fruit juice and a pinch of lo-salt for ghetto kind-of-isotonic energy drink. lo-salt has potassium chloride as well as sodium salt so tastes funny on your chips but in the right doses better than normal salt in your drinks imho.
+1 for poundshop dustpan brush, see also poundshop kitchen/scrubbing brushes and toothbrushes.
Plumbers silicon grease for disc brake pistons, and the pointy end of a slender ziptie to apply it: even a little pot for a fiver will last you forever.
Milton for camelbaks, and a track pump and wodges of kitchen roll pumped through if you ever need a fun way to de-mould a camelbak tube (but sterilise it afterwrds too)
Sugru and shoe-goo for a multitude of bodges and repairs.
Duct tape wrapped many times round a credit-card sized card, makes a tyre boot and also useful first aid for helping keep anything really gapey closed on your way to hospital.
Napisan as sports wash.
Babywipes clean all sorts really quickly.
Not really a non-bike product, but disc brake cleaner is also great for getting your rim brake rims even cleaner than you though they were when you washed your bike, improves braking performance ime.