[gnickgnack2]

I’ve heard salty, uncooked oatmeal and a pan of fresh water...bloats em up real good.

[Do_Tar]

I’ve used dry flour and baking soda, 50/50 mix, on a paper plate.
It’s nice because you can see if they’re leaving tracks in it.
Pretty sure it’s pet and child safe. Though it may make your dog fart? (Rats and mice don’t fart. That’s why they bloat.)