Do you put sawdust or hay in an indoor rabbit cage?

Rabbits should always have access to hay* but not just any hay will do. Hays sold for farm animals (horses, cows) often contain legume hays (like alfalfa or clover), and these are not healthy for most adult rabbits. On the other hand, hay sold for pets often sits for months on the store's shelf and isn't fresh and healthy anymore by the time you bring it home. Consider the following guidelines:

Hay should always be fresh: it should be greenish and sweet-smelling, the bag shouldn't be full of dust or little broken bits, and there shouldn't be any mold or rot.

Adult rabbits eat grass hay. Timothy is the most common grass hay but there's also brome, orchard grass, meadow grass, Bermuda grass, etc. Oat hay is also good but it's a little high in carbs, so it shouldn't be the everyday hay but it's fine as a treat. Alfalfa (and other legume hays), however, is not recommended for normal healthy adults because it's so high in protein and carbs that it can make the rabbit sick.

Baby rabbits, pregnant/nursing rabbits, and sometimes underweight/sick rabbits eat alfalfa hay or an alfalfa-grass mix. These rabbits need the extra energy provided by alfalfa.

* Hay is high in fibre, so it keeps the rabbit's digestion healthy; hay is also the exact right kind of fibre that keeps a rabbit's teeth worn down (not all kinds of dietary fibre do this); hay also provides the rabbit with something to graze on, which is important for their mental health and general happiness.