I mostly use aspen Sanichips. It keeps humidity well and acts like cat litter. I spot clean that when needed and usually a full cleaning once a month.
I have some on paper towels when I run low on aspen or encounter mites. I hate it cause I have to change it every couple days. My kids spill there water a lot and paper towels just don't handle that like you'd think.
I've tried newspaper and that was just an overall bad idea. They'd just get under it or move it away and do their stuff there right on the naked tub. Thereby defeating the purpose of having substrate in the first place.