Encrypting a string of arbitrary size with a big integer
When you use Diffie-Hellman key agreement protocol, both sides reach the equal keys. Then, you can use it with private key encryption methods, not public ones (like AES or one-time-pad).

One philosophy of public key encryptions is for avoiding key exchanges or key agreements. They are also called asymmetric encryption methods, because the sender and the receiver of the messages have different keys.