This is the part you need to do the least amount of worrying about, simply because the choices aren't incredibly diverse. Generally, any aquarium gravel will do well in this case. 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 in the aquarium.
Decor There are all kinds of aquarium decorations available in pet stores for your tank. Plastic plants are really good, and a place for the animals to hide is nice too. Ask about it at your local pet store. The real plants really add a nice natural touch to the tank environment and they are adored by the frogs as well, but keep in mind that it tends to require a lot more attention to cleaning and filtration. If you have aquatic frogs you may have considered housing some sort of fish in the same tank with your frogs.