As already mentioned it is not safe to place fake/artificial plants into a hamsters cage - as they could possibly chew them and ingest pieces which could be dangerous.
I also wouldn't recommend using live plants inside the cage either but a safe alternative would be sprouting seeds (this thread is really useful -
Sprouts three ways!). You can sprout some of your hamsters favourite seeds in dishes outside the cage once they have grown then they can be placed inside your hamsters cage for them to enjoy nibbling on them. My hamster really love these sprouts although they don't last very long so wouldn't work as decoration for long but more of a treat.
Forage mixes can be used as decoration scattered on top of substrate. Another thing you could be instead using instead of actual plants is to use millet, oat or wheat sprays standing up in the cage instead.