Are there any secure ciphers you can use without a computer?

I have some kids that like encryption schemes such as the Caesar cipher and the Vigenère cipher. I would like to teach them something that's not easily breakable by todays maths and computers, but I want them to be able to use it just using pen and paper.

RSA probably isn't the way to go, since specially the prime finding part is quite intensive.

A symetric cipher like AES would also be fine, but the current AES standard requires lots of rounds of calculations to be effective.

One-time pads are nice, but not really a proper cipher.

Are there any cute modern schemes that are really cheap, but still have a reasonable amount of security?

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I also found this thread: crypto.stackexchange.com/questions/844/… –  Thomas Ahle Oct 17 '12 at 20:17

@ThomasAhle -- no, there are as many keys as there are ways to shuffle the deck of cards. That's about $2^{237}$ different keys. And of course people know the algorithm: it's described fully in a published (and rather well selling) novel. –  Henning Makholm Aug 7 '12 at 17:11