How big is the space? (Pictures, please, if possible!) And what is the budget?
Bunting immediately comes to mind. That's classic carnival decor. If the budget allows, it would be awesome to drape fabric from the ceiling in a striped, carnival tent pattern.
Build a ticket booth at the entrance? Or maybe just velvet curtains you have to pass through?
So many possibilities, it really just depends on what you mean by "breaking the budget."