What's the easiest way to convert Wiki markup to HTML?

It's not really a simple problem, because if you're going to display things back to the user, you'll need to also sanitise the input to ensure you don't create any cross site scripting vulnerabilities.

That said, you could probably do something pretty simple as you describe most easily with a regular expression replacement.

For example

replace the pattern ==([^=]*)== with <h2>\1</h2>