As far as I know there are plenty of bonuses out there that don't require you to deposit your own money, in fact here on bitcointalk you can find free bitcoins in the giveaway section, I've seen plenty of prizes/contests/etc that give a small amount of free btc 10-100k satoshis for doing some really simple tasks, you can easily use that money to gamble, again risk free.
Yes in the games and rounds you can actually participate with most of the casino's that has an ongoing give away. Most of them doesn't really require you to do but just post your userID and you are good to go whenever they credit the balance.
It is risk free and it's up on how you will grow that money but most likely those amounts are bound to lose. I've said that because it happened to most users here many times losing all of those giveaways.