I'm not a pro forum person, but...
It looks like the forum already has a lot of different boards. You could add more on, but be sure not to just copy them to be like
ZU's, or any other forum. Be creative.
Some things to keep in mind while advertising are:
Why they should join.
The benefits for joining.
What they can do on your forum.
As for the flash games, why not just make a Flash Game board, or just a Flash Game thread?