The moss balls are great, as long as they're not too small. Just make sure it's bigger than an adult axolotl's head.
As for other plants, I tend to go to the store, see whatever they have and choose the more sturdy plants, with long roots. They have a much smaller chance of getting uprooted by the axolotls.
If you're unsure, just ask the shop owner what plants do really well in cold water and are sturdy enough to withstand getting hit by something (like axolotl tails...
)
For rooted plants, what also helps is placing them in strategic locations. For example; a small indentation in driftwood, or in between two rocks (just don't smash the plant of course)... this prevents them from being uprooted
Floating plants are really good in axo tanks too.
Oh, and don't forget the key rule... plants need food too. When you want a lot of plants, get yourself a decent fertilizer. Ask your aquarium shop owner what brand would be best. I myself use EasyLife ProFito.
That takes care of the plants...
The decor you can basically choose for yourself, depending on what you like. You can place rocks, but just make sure you clean them thoroughly before placing them in the tank, and make sure you check if there's any "orange bits" on them... that could mean there's some kind of metal in the rock. Don't use those.
The safest bet for rocks is still buying them in a pet/aquarium store, but alas these are more expensive...
Furthermore, you can use driftwood (just make sure you either soak it or boil it before use). Each piece of real driftwood is unique, and can create a beautiful natural effect.
If you like the unnatural kind of things, you can buy regular aquarium decorations (castles, statues, little diver, etc).
But most of all, make sure your axolotl will have places to hide. It can be a flowerpot or vase placed on its side, a fake hollow tree log you buy in the store, a piece of driftwood that offers shelter, puzzling a few rocks together to make a "cave" (make sure it's sturdy though)...
If you need inspiration, just look at
this thread.
