A large enclosure is only the start. Rats naturally spend time searching, climbing, digging, gnawing and interacting with other rats. Human attention cannot replace compatible same-species company.
Offer different activities
Scatter part of the dry ration in clean substrate, hide it in paper, add deep digging boxes, safe branches, hammocks, tunnels and cardboard. Start puzzles simply and distribute valuable resources so shy rats are not excluded.
Train cooperative care
With rewards, rats can learn to enter a carrier, step onto scales and follow a hand target. Keep sessions brief and voluntary. Provide supervised free-roaming in a rat-proof room, protect cables and gaps, and never lift a rat by the tail.