There are play gyms you can hang on the ceiling.

Cheapest way is to make them. Wood and PVC are two popular materials used, because they're fairly cheap and easy.
Not to mention you can also expand on what has already been made, so yyou can always improve on it if you don't feel completely happy with it.
I've also seen nets hung on walls and ceilings for birds to climb on. Would be good for hanging toys, too.
A good sized tree stand or play stand that you can fit multiple cockatiels on might be a good idea too. These too can be made.
If you ever short on ideas, you can look up bird rooms on the Internet. Here's one of my favorites:
