I have to agree with everyone that games will retain members longer.
What we always recommend is to hold contests with your members, like high score of the month gets such and such prize or a free month membership.
If you have multiple sites you can setup competitions between the sites to see who is best. Or you can even setup competitions with your competitors.
The big thing about games is to build it into your site so it becomes some type of competition.
We offer public, private, competition and custom top score boards with our games.
http://www.adultarcadegames.com/webmasters/
Don't like our models? We can even use yours.
Hugs,
Danielle