I'd say go with a real bike shoe of some sort and just not put any clips in. I'm pretty happy with the sixsixone launches that I picked up a couple of weeks ago. They're BMX shoes that look a lot like a skate shoe, and they're pretty OK to walk in, too. If you ever decide to go clipless, you'll already have the shoes, so it won't be as much of an expense. Most MTB or BMX shoes probably won't have that much of the cleat sticking out, so they won't be too bad for walking.