I'm already familiar with coinbase and Gdax but they only cover four coins at the moment.
What is a good exchange where I could buy and sell alt coins like Ripple, Dash, Monero, and Cardano?
Ideally, the fees will be zero or very low. Obviously I want something secure as well.
I think bittrex will be your best friend, it has the 4 coin you stated above and the volume within exchange itself is huge so it's easy to trade there, the fee is quite cheap in my opinion and the interface is easy to understand. There's a also a lot of indicator that you can use to trade so basically bittrex is full of features.