I'm absolutely terrified of letting my rat free roam on his own with no leash or anything on. We live in a small apartment with many many electrical cords and wires, two cats, tons of places to get stuck under and in and so many other places to get hurt in.
My cats won't hurt him since Taloose is a sweetie and has been face to face with him several times and has nary even moved other than looking at him, or a sniff. Layla, the other, more aggressive and grumpy cat (she's got hip displaysia) is actually afraid of him.
But I'm still afraid he might run under or inside something because he's afraid of them.

We're moving in 2-3 weeks into a new apartment with a loft, and I'll be living up in the loft with my bunk house (top is a Full bed, and below is a desk with a narrow shelf) and his cage will be relocated to my desk, and I can convert my desk into a ratty play area...
but I'm not sure that's enough!
This is the first rat I have ever owned for longer than a few weeks (two others; one was extremely aggressive and I only have one cage so I had to give him back rather than separate him, and the other died of mycoplasmosis shortly after I got him) so I'm not entirely sure that a ratty play area would be enough to keep him entertained and healthy.

He's a lone rat but when we move I'm getting him a cagemate.