What kind of decorations are OK to use?

The tank setup I have is kind of bare, and I think it needs some more decorations for the snake to have some cover if he moves about. I'm thinking of those fake leafy vines I've seen in the pet store.

What kind are safe to buy and put in his cage?

The ones in the pet store (in the reptile section, usually) are fine. A cheaper alternative to clutter up an enclosure (but much less pretty) is just loosely wadded newspaper. For more privacy without just stuff lying in the tank, you can also black out the sides and back of the tank either with plain paper or you can usually find the 'window cling' style decorations that have leaves or trees or whatever on them.

Are the fake plastic plants from craft stores (Micheals, Hobby Lobby, etc) OK to use?

I use the fake plants from petsmart located in the reptile section ;)

For the most part any fake plant is ok. I have criteria I use I can share them if that helps.

If rubbed hard no colour can come off.

The wire must not be poking through.

It must not have very strong smell.