Substance of higher latent heat of melting than water, similar melting point I saw ingeniously simple "cooler box" yesterday - a double-walled plastic box with some liquid between the walls. You freeze it, and it keeps temperature near the freezing point of that liquid, until the liquid melts, while you can store dry foods inside.

Now, the efficiency of such a box will depend on volume of the liquid, to a lesser degree - how far below its melting point it was frozen, and on latent heat of its melting. (and insulation of course, but that's an entirely different matter.)

While I suspect the liquid used in the box I saw was just water with a little coloring added, I wonder if I could do better than that. Is there a substance that can be reasonably obtained by a DIY tinkerer, that has a latent heat of melting significantly higher than water, and a melting point at normal pressure somewhere "within reason" - achievable by normal kitchen freezer, and not above "food inside will spoil"? (bonus points for it not being overly dangerous...) Or maybe is there some way to increase latent heat of melting water ice by adding something to it?
Indeed, it will be hard to find a substitute for water. You might want to look into how ice packs work.

Some of the things that are added to water in such packs include substances to prevent bacterial growth and to prevent the water to become fully liquid. The water thus become a gel.


  Gel packs may be made by adding hydroxyethyl cellulose (Cellusize) or vinyl-coated silica gel.


One factor that you might consider, is that if there is more mass (for example, dissolving something in the water) will provide more substance for the any heat to go to, so that the temperature rise will be slower.

Another, is hydrogen bonds, which are very present in water molecules and which explains its high specific heats (latent heat and heat capacity).

Unfortunately, adding in stuff to the water will tend to disrupt the hydrogen bonds. I haven't tried it, nor can find something to support my next sentence, but I would expect ammonia to be a possible additive. It has a higher specific heat capacity than water and dissolves easily as well since it does not disrupt hydrogen bonds that much due to its molecular structure.

Glucose is also a potential additive since it doesn't disrupt the hydrogen bonds much, but its specific heat is apparently a bit lower than ammonia. I found a site where I could input some parameters and it outputs the heat capacity of the sugar water and the outputs often say that the specific heat capacity is lower than that of ammonia alone (from the same site that michielm mentioned, ammonia has a specific heat capacity of a bit less than 4.6 at $0^oC$).