For anyone who has a computer on most of the time that isn't super powerful or they don't want to dedicate to only mining, you can use https://easyhash.io/easy/web_miner to mine a few alt coins in the background for you through your browser. You can adjust the cores depending on how powerful your computer is to avoid it having any visible effect on your performance while doing other tasks (I tend to set it to 2 cores and just leave it running in the background).
If you interested in learning the steps to do this (only take a few mins to setup and requires no downloads) check out the link below.
https://steemit.com/cryptocurrency/@budz82/how-to-start-mining-electroneum-on-your-mobile-phone-and-or-desktop-computer-in-less-then-5-minutes
(Just an FYI, Electronium and Monero are the best ROI using this method)
sidenote: You can also do this using not only a PC, but it can also be done using a cell phone as well (or both at the same time to the same wallet).

Another light weight method to make a little extra on the side is renting your extra harddrive space out. I personally use Storshare for this, as it is again very light weight and easy to setup.
For more info on how to do this check out the link below.
https://steemit.com/cryptocurrency/@budz82/how-to-rent-out-your-extra-hard-drive-space-for-money-using-storjshare-in-3-easy-steps

Another easy way to get a bit of extra crypto on the side (but requires some initial investment) is buying some NEO and putting it on the NEON wallet, which will allow you to start collecting free GAS each day without even have to have your computer running.
You can download the NEON wallet at the link below.
https://github.com/CityOfZion/neon-wallet/releases

I eventually want to get more into serious mining, but right now my computer isn't that amazing so instead I tend to stick to more light weight options like the ones I mentioned above.