Hitting it with a hammer works. So does drilling a hole in the case. The instant the inside is exposed to normal dust and air, it's done.
I would not expect exposure to air or even dust to damage it, as long as you don't have the heads out on the platters.
It may require a trip to drivesavers to open and clean out first though.
It all depends on just how paranoid you are though. The grinder trick is guaranteed, but even drilling a hole or two in the platters won't stop someone with a lot of money. Banks have a variety of methods, but one I've seen is placing the drive on the bed of a drill press and dropping a few 1/4" holes into it. That takes care of just about everyone. A small amount of your data is physically gone, and the rest would require a great deal of effort to recover.
I think in most cases a simple one-pass zero is sufficient. I've seen two different tests where a group took a handful of drives and did one pass erase on them and then shipped them to several of the more mundane data recovery places, and not a single one was recoverable by them. But if they'd have had a few hundred k to drop they probably could have sent it somewhere to get the data back.
If you can't physically access the drive, and it's not an obvious head crash, there's about a 75% chance drivesavers etc can recover the data. This is based on a few dozen of our customers having to send in their failed drives for recovery. So if you can't zero it, you really should do something physical to it if it had seriously sensitive information on it. Take out the platters and smack 'em a few times with a hammer or gouge up the surface with a screwdriver, and that will reduce the odds of recovery to allowably small. It will cost around $2-5k to recover the data if you just throw it away without damaging it. If the data isn't worth that much in the right hands, then you are probably wasting your time taking it apart, it just wouldn't be worth anyone's effort.