CPU Mining?
There aren't any profitable options for you.
But just for fun or experience, you can try Monero or Electroneum.
If you didn't really need the rewards often, run a full node ETN because it's small at this date.

Join a pool for any of both options if you need a little bit if profit.
Electroneum: easier to mine, price might grow in the future.
Monero: the price is better today.