I am always interested in learing a new language and brushing up on those I know too.
I read somewhere that Mindsnacks was a great app.
So, I uploaded Mindsnacks French for free, it comes with a free lesson, but for $4.99 + tax you can receive all 50 lessons, so I bought it.
Then, they went and had a sale on all their language apps for a one time price of $17.99 + tax...so I got all the ones on sale.
The lessons are in the form of games, most resemble flash cards, but it does keep your mind active and challenged.
So far, I've worked through French and am working through Japanese.
I've also tried a lesson in Mandarin and it is killer!
If you try them, let me know what you think.