easy access day-care
I suppose some/most may already know this, but I haven't seen it posted anywhere. If it is, forgive me. There is a much closer place to waste steps waiting for an egg to hatch than the bike path, which I've seen reccomended often.
Stand in font of the "Mauville City" sign in (guess which) City, and run/ride left. You'll wind up in front of the battle tent in Verdanturf Town. Far, huh? Go right, and you'll hit a rock a bit right of Mauville. This is a long line to run, goes perfectly straight, and contains 135 steps. Also, if you're trying to get eggs, you'll be running right past the place, so you can check up on the ol' sneaky day care man at each pass.