How do I enable Wi-Fi-only email push? Is it possible to get iOS's email push feature to work only when I'm on Wi-Fi?
As far as I know, not with the stock options. You may be able to do this on a jailbroken phone if there's a jailbreak community app for that. Short of that, you'd have to make one yourself.

Otherwise, you're down to changing these settings manually when you're away from WiFi.