Lojban

Lojban is a constructed language. Its grammar is based on the principles of logic. The syntax is unambiguous, but you can still semantically be ambiguous if you want. You can express complex emotions or attitudes too. Learning Lojban is intellectually stimulating.

http://en.wikipedia.org/wiki/Lojban
http://www.lojban.org/

Or then learn a real language and talk with many different people from other cultures. Very intellectually stimulating.