The right thing

I'd grow another eye because I'm blind in one and I think it would be absolutely amazing to see out of two eyes

Does the extra body part have to be one I already have?

And can I restore parts evolution did away with?

I'd grow a brain so I can live another life in there

If I could make it disappear again anytime I want, I'd grow a penis. Simply out of curiousity.

Does enhancing your brain counts?

I'd grow a dragon tail. Because just one wing is useless and a dragon tail is big, fat and squishy and useful for slapping bitches

I'd grow a massive tumor in my brain and die immediately