There is a website...
... which offers free language lessons structured similarly to Rosetta Stone software. I believe they teach English, German, French, Spanish, Italian, and Portuguese. Choose one you don't know, and learn it. My fiance and I are considering going to Lake Constance in Germany for our honeymoon and I am currently making great progress learning German. I hope to be at least conversational one year from now, but I don't have 8-10 hours per day to work on it. An hour per day on something like that for 6 months should make you pretty much proficient.
Skills that are relevant to your job but niche-y might come in handy. The professor who is going to be guiding me through my MA is on sabbatical next semester, and I believe he is planning on learning some new software for the cognitive research we conduct in our lab. He could just consult with someone with more computer software knowledge (programming, in particular) but is instead choosing to familiarize himself with it. These skills make you more well rounded at your job.
If your only goal is picking up a skill that will make you extra money, computer skills are a good way to go. You can audit classes at your local community college for next to nothing that will teach you the basics.
Please let us know (via this thread, maybe bookmark it and bump with updates?) what you choose to do and how your progress, and what results you see. I would be curious to know. Thanks!