I'm a sucker for photo editing apps and have two whole folders of them on my iPhone. One of my favorites is Afterlight mainly because it's simple to use. I can apply lots of filters on top of each other or use it just to make basic enhancements. After you purchase the app, you'll have access to all features except for scene clips. If you'd like those it's an additional $0.99 in-app purchase.
If you simply want Afterlight as a simple way to edit photos on the go, you probably won't have any interest in scene clips but if you do, you've got the option all in one app