We have weighted hangers that sit on top of the mantle with an L-bracket hanging down that has a hook.
Command strips w/ hooks
Stocking hangers . Or command hooks. Stocking hangers are cuter. I vote for cat ones but heres some others just in case.
You can use a stocking clip:
or one of these: (there are MANY designs... check review however... they need to be heavy)
I screwed in cup hooks under the mantle. I also hang them from the fireplace tool set.
Command brand Hooks! They come in various sizes, shapes, etc. and each style will list how much weight the product can hold! here's a link for you.