My girls like a shelf I put at the very top of their cage that I clip a sleep sack to for sleeping in a giant pile. They love to shred paper and fill up the sack with it. They only like hammocks for eating in-they drag their food up to them and eat there like they are their dining rooms. They love wheels, wooden chews. the lava perches they like to sit on and chew. They use ladders, ropes and small plastic baskets to climb from place to place in.
Boys-They are more diverse in sleeping habits-some like to sleep in plastic hanging tubes, some like plastic balls (with out the lids), some like hammocks, some like a blanket on the floor and several love the wood flat platforms my hubby made to sleep on. They like all toys-wooden chews, plastic balls, basically anything small enough to hide somewhere. They also use all the climbing stuff and shelves. None of the boys like wheels.
Everyone likes their litterbox, for it's intended purpose and for storing food and toys in and sometimes blankets. Boys like to use their pee rocks (they like to pee on them and they seem like a good place to sit on to eat), girls dont.
I have the plastic feeding bowls that attach to the cage, and the large ones they like to sleep in sometimes too. I use them them as "toy boxes".
We go through a TON of wood chews. We finally cut down a large branch off a pear tree (that we know hasn't had anything sprayed on it in at least the last decade anyways) and made a huge batch of small blocks out of it, enough to fill a big storage bin. We dyed them with food coloring, soaked em in vinegar and then dried them slowly in the oven to make sure no parasites/mold/fungus or whatever in it. We also made little round platforms for them with it as well to both climb on and chew.
Any cardboard boxes we save as well for both chewing and as hides. My boys like small wooden huts to sleep in as well, my girls don't seem to care about those.
Water bowls are a source of entertainment-they like to wash their faces in it and the boys like to sit on the edge and hang their parts in it...I don't ask questions. The girls require bowls under their water bottles cause they like to stick their hands in them so the water falls over their heads.