Anything you care to tackle! Provided you have some other vehicles with you with sufficient recovery gear to get you out of stucks or help you past/over things you can't do in 4x2, you're free to try things and learn for yourself what your X can do. It's the only way you'll really challenge yourself and your X.
We took a 4x2 without the limited-slip rear diff on a trail a few months ago, it was his first 'wheeling trip. He took a bad line up a heavily rutted firm dry hill and got stuck, and nearly threw in the towel right there. We convinced him to let us pull him over and keep going, and he was glad he did. He had a blast up until there was a rocky hill he just couldn't conquer and a pull up it would have taken too long. He ran the same trail in the other direction (down that hill) later that day though.

The 4x2 will surprise you as long as you keep pushing it. Just have the help you might need handy and go for it!

Brent