They don't have to be smooth like glass, just not rough enough to cut.  My frogs have some resin tank decs that are pebbly to the touch, and it hasn't hurt them a bit.

Things to consider:
* No trap-and-drown hazards.  This means no domes or towers or tunnels without exits at the top, and no moving parts that could catch a limb or flipper.  Also, no tangly plants - broad leaves are better than fern-looking leaves.
* No scratch-and-cut hazards.
* Aquarium safe.  Anything that isn't aquarium safe (or food safe inside and out) may leach harmful substances into the water.  No cobalt glass, driftwood, shells, etc.

That's all I can think of.  Meet those criteria, and your frogs should be fine.  I used to have a lot of trouble finding things to meet those criteria (which is why we all used to resort to things like terra cotta pots and PVC pipe -- those are still fine, of course).  Now I find a lot of stuff at PetSmart, meant for aquariums, that is frog safe, so I'm not as interested in creating my own tank decs as I used to be.