Klingon. The alien tongue from Startrek. Or Sindarin. The elven tongue from Lord of the Rings.
Other than that, depends on what you consider fun.
I recommend learning some minority language. Any tongue that has less than 100000 speakers. Like Scots Gaelic or Pennsylvanian Deitsch... so you can do make it a campaign to fend for a waning culture.
Or anything that sounds weird to you. Finnish is good there. Sounds weird to almost everyone except Finlands people....
Russian is a crazy tough but really cool language, right now im learning Swedish which I personally really like. French is a beautiful language too! But if you are going to learn a language i would recommend using Rosetta Stone! its sooo easy and makes learning the new language fast and fun.
i would say Mandarin because its so commonly used now a days. Personally i agree with the people who said french. Its such a beautiful language. bonne chance! :)
Räto Romanisch (original "Swiss" lenguage") only 30 000 people on the world speak it, i'm proudly one of them :).
French sounds really cool, and I love being silly and speaking French with my buddies.
I enjoyed learning french and italian more than the other ones ive learned :)
chezch. (sp) i know how to say im going to smack you in czech.
i think Japanese is a fun language to learn
depends what you like to hear...:)