Not nearly enough. J'onn's greedy, you see; he wants powers you've never heard of, powers that only frumpy middle-aged Manhattanite sci-fi geeks in the 1950s could imagine, and so many of them that he can use them once, then discard and forget about them, like disposable towelettes.
He can draw gold from seawater with his mind.
Okay, I'm sorry; I know everyone's already seen this panel. It's from the Origin of the Martian Manhunter. But what better way to start Martian Manhunter Week than with this perfect example of his absurdity? But that's nothing...
He can create ice cream cones out of thin air by summoning powers from the void.
Next time someone at your neighborhood comic book store is talking about how dumb Jonn'z weakness to fire is, pipe up knowingly with, "Sh'yeah, I mean, why doesn't he just recreate the atom combinations of a giant ice cream cone using the powers from the void of space and use it to smother the flames?" That'll shut everyone up, and all the girls will drop their copies of Local and run over to hang on you admiringly. Works every time.
J'onn can make your clothes wash and iron themselves through substitutiary locomotion.
They aren't exactly proud of it, but everywhere other than Earth, Martians aren't manhunters; they're launderers. Yes, everywhere from Rann to Rimbor, there are little mom & pop Martian laundries, as well as Martian take-out places famous for their "Summoned From the Void"-flavored ice cream, whose owners go home nightly to Martiantown on the edge of the city. Once a year, tourists visit during Martian New Year to enjoy the parade of water-breathing dragons and watercrackers. And speaking of water-breathing...
He can breathe underwater.
Must remember that for my Aquaman-themed Heroclix games on the water maps.
Really, J'onn; would it kill you to put the helmet on, in case someone sees you? Show-off. Actually, he's probably just being greedy: "As long as I'm down here, I'll fill my helmet with gold dust I extract from the seawater with my mind, then make up some lame, last-minute explanation for it when I surface to rejoin my colleagues!"
He can magnetize metal through sheer concentration.
He can grow to gigantic proportions (furthering my Tony Robbins Theory, I might add).
Now, that is a power so useful and impressive in battle, that he surely will use it again and again. What, never? Well, hardly ever. Wizkids, please remember to make a giant Martian Manhunter figure, or at least give him 'Enbiggen' as a special power. Oh, and, conversely...
He can shrink to mini-mate proportions.
If I had to choose a sound effect for shrinking, it wouldn't be "Ka-zoom". Maybe the wizard Shazam gave him the power?
Naturally. As if the god-like Martian Manhunter wouldn't covet the power of people like Bumblebee and Doll Man.
There are more powers; many more, including such lulus as the ability to scare sharks with his appearance, "atom vision", and "around-the-corner vision" (because x-ray vision is not enough, never enough). But, honestly, I don't have the strength to scan them all. So, before I leave you either to ponder what they are or to simply go buy the Martian Manhunter Showcase, I give you one final power...
He can violate the Fourth Wall at will.
Tony Robbins, Grant Morrison-- the Martians are everywhere!
P.S. Remember when I said I would spare you?