There is an Opposites app that gets pretty advanced on vocabulary for iPad.

Also Raz-Kids from AtoZLearning.com offers a huge collection of leveled readers (Aa-Z) that can be read or listened to and has comprehension questions at the end. You can also record yourself reading and play it back. You earn stars for all of these activities and can design and build a cool rocket. If you buy the subscription ($99 annual), you can download the Ipad app for free.