+1 to that. The most "useful" thing you could do would be to seek depth, not breadth: Keep working on your mediocre Spanish until it becomes fluent Spanish. There's an absolute ton of Spanish speakers in the USA, to say nothing of the rest of the hemisphere.
(By extension, another useful thing you could do is just continue practicing ever-more-advanced English. :) But that presumably fails the "fun" test big time...)
From there, if you've ruled out Chinese (native speakers everywhere!) and Japanese (inspiring manga and anime everywhere!) as too hard (which is probably wise), I think you're down to:
- French (folks on the web rave about French in Action; lots of speakers around the world; great movies and literature; and the food...)
- a language spoken by some nearby foreign friends who will be pleased to practice with you occasionally (in my case: Chinese, Polish, Hebrew)
- a language spoken in a country you really want to spend time visiting (hint: Italian)
- Latin. Intellectually interesting. Relatively easy to study without a teacher: Your accent will almost certainly suck, but none of the native speakers will ever care, because they're all dead! And you've got thousands of years of classic literature to appreciate:
http://www.amazon.com/Harrius-Potter-Philosophi-Lapis-Philos...
http://www.amazon.com/Harrius-Potter-Philosophi-Lapis-Philos...
This has been available for a while though.