Some play cards, play board games and some even "if allowed" work in the kitchen helping prepare food and others do some cleaning of the prison.
Depending on your custody level you might be let out of your prison cell and out of the building and get to go outside in the rec yard for awhile to get some fresh air and walk around.
Someone who committed murder though and is a real danger might not get to go outside in a rec area. And if they do they'll most likely be in a cage outside. But not really outside.
Watch some prison shows on youtube and you'll get to see what prison is like.