Mine have good quality catnip toys down all the time and in the summer I have giant pot of plants.
They help themselves when they want and thats it.
There is no danger as long as you dont try and take away their fun...........and then you may get a clawed warning
The bestest toys are the green soggy ones and the old mice are loved the most.
I make a sucky sock for Sasa in the summer,just put some fresh leaves of catnip into and old sock and knot the top, keep refilling as it gets soggy and green