What I do it I write poetry to expand my vocabulary, so a journal works too! As long as it's written. I tend to read a lot, so if I come across a word I don't understand, I write it down and look up the definition. If I like the word, and want it in my vocabulary, it's inserted into my poetry.
And if you write a journal, but feel it's all complaints, well, that's why we keep diaries and things. We need something to vent to. So it's fine if it's all complaints, just make sure you're happy about stuff too and it's all good!