Ipad app for vocab building and critical thinking?

[WashDayBlues]

Hi all, I'm looking for an Ipad app for vocabulary building for a bright Year 5 boy. (I realise reading is the best way to do that, but I like to have a few educational apps on the go too). I'm thinking of something that introduces new words, putting their meaning in context, and challenges on synonyms and antonyms.

Plus, any critical thinking apps you would recommend?

DD (year 5) is actually encouraged to create her own books using book creator on her school iPad. She has made a book on coastal environments, a school trip, an opera company that visited school, classification of living things and a book about light and vision. This encourages her to use different types of language and to take research and write it in her own words. Rewriting this research actual develops the use of synonyms.