These days with smartphones is easier to study or practise Japanese outdoor than some years ago, when for example I had to take with me a little notebook plenty of references written by me about vocabulary and grammar.
Since I had smartphones I used different Japanese app for different reasons, like to practise for Noken, for studying kanjis or for having some digital dictionaries. But now there are a very useful app that I want to recommend here, its name is Takoboto, an offline dictionary so complete, you can write a word in English or Japanese, and while you are writing it different options will be appearing, trying to fit with the word you're writing. I was using another app called Jsho, it's quite interesting as well, but it's slower, you have to wait to write the word to see the results, and you have to change if you want to write a Japanese or English word. I don't know if Takoboto has its version for iPhones, maybe you have to try and see. But I thought it would be interesting to comment about it here.