You might want to have a look at cryptocurrencies like Verium or Magi. I for example have 4x Pi3 running in the basement on Verium and it works really well. No big profits but it is fun setting it up and it is running in the background 24/7 on a 5 port USB charger without any issues.

Odroid XU4 would be even more efficient with this but they are hard to get and expensive where I live so I went with the Pi3's.