Greetings,
I guess it depends, crypto is quite a wide subject, what are you going to teach? I'm not familiar with the grading system so... Do you want to implement actual algorithms? Then re implementing md5 Can be a good start! Its not that hard to do and it does look impressive... Other than that, depending on your students level you could have a in depth look at pki set up and management, or even implementing well known algorithms that are a bit harder such as rsa or current aes. In both those endeavors, having handy copies ofCrypt::Openssl::AES and Crypt::Openssl::RSA can be a boon as reference material and such.
If you decide to put yourself in a hacker's shoes (which can do for a fun programming camp) then you can go the audit road. It might take a bit of a preparation but you can grab any entry level cryptanalysis book and do easy two step classes for a bunch of well known ciphers (eg caesar):

1. implement the cypher
2. Do a cryptanalysis

if your students are very good you can also go in full cracker mode, prepare your course by implementing some algorithms (such as md5,aes, rsa) in a sloppy way, thus introducing a flaw only YOU know about (or a backdoor, depending on the pov) and then look for it with your students.

On that topic, something else you can go about implementing would be a random number generator, I'd advise implementing two, one cryptographically strong and a weak one. Then you make the students use both and study attacks against both, thus demonstrating why using low entropy sources is a bad idea.

Food for thought, food for thought... Hope I have helped.