kitty litter IMHO works best and is the cheapest to use. Anything with food in it brings critters. Sand you can end up with sand fleas.
When making mine I double bag the stoppers so nothing leaks out.

just be sure to use the unscented kitty litter :) the scented stuff is a big turn off IMHO.

Yes......do not put anything edible in a draft stopper, especially if you live in an old house, and especially if it's in the country.

I used white rice- very cheap- but I also have a cat..... No critters in this old house!

I just made a bunch of these insulation sleeves. I used a box of old colorful sheets and cut them a couple of inches wider than the window and about 14 inches tall. I used Stitch Witchery to make a seam and turned them inside out. I then took roll insulation (a roll is pretty cheap and the kind with paper backing is easier to thread through) and cut to the same width as my sleeve. I cut the insulation in two lengthwise then folded the cut section over lengthwise so that the paper was only on the inside of the insulation. I then fished that through the sleeve. And wow, I'm a little surprised how well they work. And pretty cheap too.

Don't bet on the cat keeping the mice out! I've got three cats all young and healthy. I still set my mouse traps and I still catch mice because the cats don't do a good enough job.
My house is old (180 yrs old), it's sits near fields, woods and farmland. I don't catch many mice up in the house proper BUT in the basement the traps get tripped on a regular basis. Just cause you aren't seeing them does not mean they aren't there.

The mice get into places that cats can't - such as closed kitchen cabinets and closets, the little troublemakers. They can slip through a hole smaller than a dime.

I'm about to make some for our house. I was going to use canvas fabric leftover from another project, polyfil stuffing to give it some body, and those weighted plastic pellets they use for making teddy bears, dolls and beanbags to give it some weight. That way, there's nothing organic to attract critters, and the whole thing can be thrown in the wash if it gets dusty or damp.