First of all great job on the traffic, 20,000 DAUs is really good. I have a lot of experience in the mobile game industry, specifically in Word, Card, and Casual titles. My answer is based on that experience.

I have a few ideas on how you can monetize your website. I will put them in order of what I think would be most effective for revenue and retaining users.

1) Incentized Ads - This is a great way to use ads without turning your site into an "ad kingdom." An example of this would be to ask the user to watch a video ad in exchange for playing the game. For example they enter your site and start playing the game and when they are done you ask them to watch a short video to play again. The advertisers will pay you for every video play and the user takes the action.

2) Subscription - This model is best used by Netflix. Get them to play the game for a trial basis and if they want to continue they pay a small subscription free.

3) Donate - This is the least lucrative. Wikipedia uses this model and asks users to pay to keep the service going.

If you need more details please let me know.

Take Care,

Jonathan