You should probably try to pool-mine Monero (XMR) or Ethereum (ETH).
Pool mining is where a bunch of people mine to a common managed pool to increase the chances of finding a block and splitting the profits as opposed to solo mining where you'd get the full reward for finding a block but has lottery type odds.
My kind of recent experience: with an old Radeon HD7850 I could achieve ~430 H/s on Monero. An imac's i5 2.7ghz gives ~25 H/s per core, so about 100. With a setup of around 530-600 H/s I managed to mine a whooping 0.1 XMR in about 10 days (about $4.40 at current prices). Fun fact: the pool has a minimum payout of 0.3xmr, so I would have to mine another 20 days to get paid.
Still, monero is one of the more cpu/gpu friendly cryptos, so if you have some good graphics cards around, you can probably make some profit. I've also read somewhere that ethereum can be profitable with gpus but didn't get around to testing it.
Although you'd probably have to benchmark and know your hash-rate for the different coins, if you know that, here are some useful online calculators where you can input that along with your electricity cost and see how profitable it could potentially be:
https://www.cryptocompare.com/mining/calculator/
https://www.nicehash.com/?p=calc