I'd by lying if I said you could find something useful to contribute in just one evening's work, but if you have a few evenings you could probably pick a core library class, see what is currently missing in it, and fill in a few gaps.

You'd need to understand Java and Ruby, but we can help you with the rest.

Join us #jruby on Freenode IRC, or https://gitter.im/jruby/jruby.