Choose the game that interests you and play itfor free.
Complete the quests
Every game has some quests for which you will receive Soul Gems.
You can exchange your Soul Gems for Steam Wallet top-ups, game keys, CS:GO Skins and other rewards.
Hey! Yeah, you read right: Bethesda is giving away the Elder Scrolls Morrowind as part of The Elder Scrolls 25th anniversary.
Go to bethesda.net and check it out.
On a side note. Please let me know if anyone can redeem the game. I have created 2 accounts and haven't been able to log in due to "Incorrect username/password", and I've changed my password 4 TIMES already. And even cleared cookies and all that stuff.
Thanks, and have fun!
EDIT: I found a solution! The site must be overloaded or something, I can log in normally on the Bethesda Client.