I have a humor site with over 300k unique visitors a month, mostly USA traffic. I've been trying CPA offers on it with limit success. I've tried the following things this year:
T-shirts perhaps?

Have you tried IQ Quiz type offers and dating offers?
Look at what is being advertising on your adsense ads, see if you can pull any inspiration from them.
Also look at what your competition is doing.
My friend is actually in a similar spot as you he just got his site hitting 15,000 uniques a day and is struggling to monetize it. I'll let you know if he hits something sweet.
AdSense will buy anything 
I strongly suggest you go after brand advertisers. Offer your inventory with some of the bigger exchanges where brand advertisers buy. Probably won't fulfill all of it but even 50% fulfillment at $2 CPM will make up for it.
content locking maybe
Content locking will kill you traffic especially if its return/direct traffic. If it's search traffic and you not to bothered about people not getting pissed off then content locking = bank.
well I would try interstitials...and set them up your self....kinda like deviant art does.... most people wont consider them to bothersome and you can do some testing of timeouts and the like.....just run your own stuff set up a co-reg or 2...maybe some games.... technically its not incent so most offers are open.....
but what I would realy do first is find your demo...like the were to's and who fors like were they go before and afterwards....(ie... http://www.quantcast.com/measurement/why-get-quantified ...its free) get an idea of there frame of mind while they are there and why that came there in the first place....hitting them with a co-reg that is in tune with there mindset at a certain point...like at the point they get bored and are gonna bounce anyways....could be good money.....like if you know your average page views per visit is 6 pop a interstitial at page view 5.....
thems my .987 cents