Are there any simple and yet secure encryption algorithms? Being very new to C++ and cryptography, I finally managed to implement a version of the Vinegere algorithm. I would like to try something a bit more complicated. I have looked at AES and DES and others like them, but I was wondering if there is something simpler. Something that might use some of the same basic ideas as AES, but is much simpler. Some kind of block cipher maybe? 

So I am asking for suggestions for a block cipher that is much simpler than AES, but more complex than a basic substitution algorithm. I have by the way looked into the method used in the Enigma-machines, but from what I understand they do "basically" the same as Vigenere.
A simple block cipher would be Threefish (p. 11-13). It's a bit more complicated than RC4 or RC5 yet doesn't drive you insane with seemingly random design choices. It is presumably secure and was designed by experts but has not yet been reviewed extensively, so it shouldn't be used in sensitive applications yet (consider it an exercise).

You'll be able to implement the key schedule and the encryption/decryption functions, and if you use the PDF I linked above, it will be an excellent exercise in following a specification.

If anything, it's the technical details surrounding the implementations that can take you forever to fix, such as endianness concerns in particular (this isn't too much of a hassle for block ciphers but it did get in my way a few times when implementing hash functions).

PS: same name lol