I think esperanto is interesting, for reasons i wrote here (a simple site): http://lukecall.net/e-9223372036854580627.html

(things like: the cheapest way for the world to talk to each other, fun, not replacing native languages, learn it in 1/4 the time of Spanish or 1/10 the time of English, and probably learn it plus a subsequent language in less total time spent, possible travel benefits with "pasporta servo" (free lodging).)