Signing up for their player's clubs will get you some free stuff, even if you never play any of the table games or their slots. That's the most free stuff you'll ever get from any casino, beyond that it's going to cost you.
You can find the RTP in the info section of a slot game. The higher the RTP, the more likely you will earn a prize. Consider the Amount: Some online casinos offer small no-deposit free play bonuses of $10. This is great, but other casinos have higher amounts, such as $25, $50, or even $100.