- If you believe that they're a punishment, then probably nothing.
- If you believe that prisons are there to rehabilitate people into being good members of society, then things like libraries, televisions, and other educational things should definitely be there.
- If you believe prisons are there mostly to protect the public from people who have committed crimes, then they should really be quite similar to life outside of prison just with the freedom to move around in public removed.
It's probably some combination of the second two. Prisons as a punishment only serves to create worse criminals who are suffering some deep trauma when they leave, which causes them to commit more crime. But also, people who have committed crimes that don't demonstrate a risk to others probably ought to receive non-custodial sentences and then prisons can be much smaller.