First of all, I would write simple things which use to do more or less the same in all languages, but all the time look different - looping constructs, string interpolation etc.

BTW, I have the problem too that, hopping from project to project, I have to deal with different languages and come back to an "old" one maybe after years. I use a webbased notesheet (in my case, I use the note sheet which come with the http://fastmail.fm/ freemail account, but there are plenty of others available which do the same) where I jot down constructs I find "interesting" in a language. It turns out that most of the "interesting" stuff often fits on one printed page, if you print it with a not-too-large font...