Overview of achievable properties: homomorphisms, non-homomorphisms.
The RSA algorithm requires more number theory than ElGamal. Thus adding ElGamal as an example is good as it basically only requires residue (congruence) classes. And the discrete logarithm problem is quite intuitive.
Public-key encryption:
The RSA algorithm requires more number theory than ElGamal. Thus adding ElGamal as an example is good as it basically only requires residue (congruence) classes. And the discrete logarithm problem is quite intuitive.