I'm also one of those who doesn't really practice, but just plays a lot. I play fiddle, lap dulcimer, banjo, mandolin and guitar. I try to pick up at least two of them everyday and get at least some playing time in. Though lately, I'll admit I've been favoring the fiddle.
Generally, I start with running a few scales just to warm the fingers up. Then I'll play some tunes I know for relaxation and move on to tunes I'm wanting to learn. If there are rough spots (and there always are), or difficult bits, I'll work them until I'm either satisfied or too frustrated to go on.
Hmmm... As I look at what I've written, it almost seems like a practice routine, though I've never thought of it as that before.