On my search of coins to CPU mine I found Primecoin, an interesting search for Prime Number chains tied into the Proof of Work for a cryptocurrency based on Bitcoin. So far the currency has broken 11 World Records in it's search, and the resulting chains are very interesting to Number Theorists and Researchers, so the energy used in it's creation and maintenance is of dual use.
Check it out if you're looking for an interesting and useful coin to CPU mine. Windows and Linux clients only though, unless you want to compile from source on macOS.