Some baby and/or toddler toys
are used for gliders by many.
The stacking rings you mentioned should be safe if it's what I'm thinking it is.
are great for hanging toys
as well as use as a chain toy themselves.
Plastic key ring w/keys and similar is good, avoid any that are filled with liquid or gel.
Some Playschool and Little Tikes stuff are also safe, check over for anything that could pose a danger, some are ok use inside the cage, while others are best for supervised "out of cage" playtime.
Was originally this
I stripped it down, left it on the base (green), but ended up just removing that as well.
The playhouses, barns, castles and the like are also used.
Also, I use a Noah's Arc toy, alternate with the tree house, they love those.
Duplo blocks, the bigger the better are also used.