Ask HN: What is the best way to revise an old language/technology?

I used to be a Java nerd back in college, but ever since then I haven't touched it. My work for the last 3-4 years has mostly been in Python and C++, and I have forgotten a lot of in's and out's of Java.

So, what is the best (and fast) way to get back in the groove with a language? Any suggestions on small projects that make you use (and hence revise) old languages that you know already?

A related question is, how do you guys stay in touch with technologies that you have learned but haven't had a chance to use in a long time?