The only thing "safe" to put under the pool for a little more comfort is the sheets of insulating foam. Sand will eventually get washed out and cause leveling issues.
I would go with a gorilla pad over the intex paper like tarp, the tarp intex gives is just junk it's not even worth trying to use it. Rocks always seem to work there way up and easily tear through the intex tarp.
I bought my 3rd intex pool new this season and I was very unhappy that they still use the same worthless ground cloth. I guess I kinda hoped I would get a better one with the ultra frame.
Even with the clunky ladder, undersized filter and paper like ground cloth these pools are well worth their price. My 20 month old son loves pool time and actually tried to swim his second time in, his older brother and sister also spend many hours in the pool. I can't wait until we get our own house, I've had such s great experience with intex pools that I will be getting a bigger one for our future home. Sorry I will stop now I'm veering way off topic.