Snapseed (Not surprisingly from NIK*) is an iPad app which has become very respected and popular, and I can see why. It has one of the easiest learning curves I've tried, and yet you can do a lot with it. (Some brief demo videos.)
Here is just the first experiment I did.
This was some of the more Effect-y settings, the app will also do basic things like contrast and cropping, of course.
By the way, I hope to take some portraits around new year, that should be fun.
*NIK has some unique filtering techniques so you can apply adjustments to only the sky or a blouse, without a lot of fiddly masking-off. It's pretty amazing when it works best, and I don't think anybody else has anything like it.