Nat Geo works great on my Android tablet and my Android phone
. Zinio > Google Play. Google Play has low-res magazines that look terrible on my high-res devices. Though, this deal isn't valid on Nat Geo anyway.
You need to go to a special redemption page, it's linked where you get the code. It's not really $50 in credits, you just get to select some mini-subs to a select group of magazines.
Didn't work when I tried it awhile ago, it needed Adobe AIR which wasn't available on tablets at the time. Google Play's res looks fine on my TF101, granted it's not a Nexus 10 level display.