The easiest option is to use gravel . It's easy to clean and comes in lots of different colors and sizes, and it's re-usable. Frogs have been known to accidentally swallow the occasional gravel stone, but this is harmless and generally the stones just pass right on through. You can also find nice stones to line your tank. The main requirement is that the stones aren't sharp, leaving areas where frogs can scrape against them and wound themselves.
Sand also works well and makes a nice soft setting.
This is especially good if the animal requires a slightly dryer environment, like many toads. It's cheap, and you can always compliment it with a few rocks. Common potting soil is a pretty good option as well. It's disposable, easy to find, easy to keep moist, and isn't expensive. The coolest thing about it is that you can plant real live plants in it and it will be really close to natural surroundings for your pet.