The World Health Organization (WHO) suggest the following mixture for oral rehydration therapy (ORT):

6 teaspoons of sugar (either white or natural sugars; e.g., honey, molasses)
1/2 teaspoon of salt
1 liter of water

Most rehydration drinks seem to have these components plus some additional ingredients (mostly flavoring, coloring and perservative, I suspect).

Personally, I would also seek a source of potassium if cramping is involved.  My go-to food for this has historically been a product called shoestring potatoes (a.k.a. papitas fritas).  The potatoes, of course, contain potassium.  Potatoes are not the greatest source ... things like raisins and almonds are much better.  However the shoestring potatoes also have salt.  (Preparation typically also involves vegetable oil and baking.)

So, on hard portage days, I generally have shoestring potatoes handy as a food source.  This item is extremely light weight, but probably needs to be repackaged into snack sized plastic bags to make it water resistant.  (Once, as I sat eating this at the picnic table at Prairie Portage, I had a young member of the staff walk by and declare that the container it comes in is a CAN!  It IS a cardboard tube with metal caps, but then I wasn't carrying it INTO the park.)

I've even given this to other members of my traveling party when they developed cramping.  The last time I did that was on a backpacking trip to the Sawtooth range in Idaho.  One of our group, an avid cycler that commonly did 100 mile races, developed cramps going over one of the passes.  I gave him a handful of shoestring potatoes and water.  Within a matter of minutes, he was good to go again.

dd