I think you have it covered, all I can think of is extra litter boxes

I found groups dont like sharing I know my Rozee hates Rocky using her litterbox

and because he dont care he does then Rozee will poop beside it if there is one Rocky poop in there LOL

and a extra water bottle Im sure you already have bedding and hammies.

As for toys the more you spend on them the less likely they are to use them LOL

I will think about this a bit more I think.