So what would you do if you were admin for a day?
(aside from deleting/turning off forum or ignoring the responsibility and posting on Facebook all day?)
Looking at the source code I would work on the meta tags to start with and work on SERP. Maybe also get some mod's installed that will help encourage user registration. Advertise with good anchor text and raise the google ranking of the site.
And my god remove some of the ad spaces, they are an eye sore. You can put ad's in once this place is active again, but build the community before you pad your wallet. You might also want to remove the front page, it's horribly out of date and fairly ugly if you ask me.
Install a user agent tool on firefox and verify what google bot can see and what it can not see in order to raise the sites SEO score and SERP.
I was able to get my old site gamerout.com into the top 5 of searches for both "gaming forums" and "video gaming forums", I sold it quickly after that, made a nice chunk of change, and the site died swiftly after I made my departure. I can not stress ANCHOR TEXT enough. If you want to rank high on google, use anchor text in the link you are advertising. This site should be a breeze to rank highly if you just put some effort in. You do not even have to pay for a thing. I have never purchased a link anywhere and my old zombie site was ranked #1, the gaming site #4 (in a horribly tough genre to place in), and every other site I have tried to rank up has got in the top 5 of the searches I targeted. And the new zombie site, which is just 10 days old, is flying up the ranks as well. I estimate #1 rankings for my keywords by Novemeber at the latest.
Like I said, this place should be easy to rank up. It has a lot of age on it (good thing), a lot of content (also a good thing), and what I can only assume to be a lot of pages already indexed in google. Just change your descriptions some, work on SEO, and advertise using anchor text.
If you need any more advice just PM me, I'd be glad to help you get this place rolling along again.