I'm making rice krispies cakes, but I just realized I'm out of gelatin and marshmallows. My mate suggested mixing normal granulated sugar with some water and it will give me the stickiness I need. Is he right? If not sugared water, what else could I use here?
There are plenty of recipes for rice krispie cakes without marshmallows in them. Rather than trying to substitute in your recipe, you'd be better off making something slightly different but that's been tested.
Here's an example from tesco. The recipe we use is based on golden syrup, but isn't online. A search for "rice Krispie cake -marshmallow", possibly with the addition of "-chocolate" will find you a decent selection.
Sugar water will just make your Krispies soggy. Anything but gelatin and marshmallows will make them taste funny, but sufficient peanut (or other nut) butter, to make the stuff stick together, plus an egg might give you something tasty and edible after baking.
A version without marshmallows would be those that bind rice krispies (or cornflakes) with chocolate.
The base procedure is to melt chocolate, optionally thin it a bit with butter and/or syrup and fold in your cereals. If you want light rice krispie treats, use white chocolate instead of dark or milk chocolate. Adjust the chocolate : cereal ratio to your liking, but use enough to bind the individual grains together.
Pouring sugar water - even with a very high sugar: water ratio - over the rice crispies will have basically the same effect as pouring milk over them in your breakfast bowl: A soggy mess within minutes.
If you had some chocolate chips on hand, you could technically make Crunch bars. I would melt 1.5 of chips per 3/4 cups of krispies. Other than that, a good basic recipe is to melt 1/2 cup nut butter and 1/2 cup honey(or maple/glucose/sticky syrup) together in a pot, then mix that into 3 cups of Rice Krispies. I personally like tahini (sesame seed) butter and maple syrup. A good basic one is honey and peanut butter, though.
Sugar water will indeed make your rice krispies soggy - however with the magic of cooking, you can apply heat to the sugar water and turn it into a toffee/caramel, which will not only bind your rice krispies well, but will set nicely so that you have a crunchy slice.
A version of this is common in NZ/Australia and is normally made with honey, butter and sugar to make the toffee
I have also had great success with a date mixture, which I find is best refridgerated, but works well at room temp too.