I think a Dark Forest theme would be pretty cool, but I think there are too many things within Action Zone that don't quite fit that theme. Drop Tower, Invertigo, Delirium, Skyrider. If it wasn't for Delirium, I think you could just confine Action Zone to a smaller area, and have a separate Dark Forest area. Maybe Delirium could be renamed and repainted to fit within the theme, but I'm not sure. But I would try to make a more distinct barrier / transition to separate Drop Tower, Invertigo, Congo Falls, and Skyflyer, and make that the new, smaller Action Zone. Maybe add a small flat ride if you can fit one in somewhere.
I would then get rid of Slingshot and put in a different flat/dark ride that fits into the Dark Forest theme. I would be ok with a retheming of Adventure Express too. I agree with others that Oktoberfest isn't too bad as it is, just that it's a pretty small area if it's losing Adventure Express and Hank's, but those don't really fit it anyway. Change Hank's into a more appropriate restaurant for that area. Keep Oktoberfest as the small area that it effectively is now, but crank the polka music for those walking through.
I don't think Rivertown needs a lot of work, maybe just a bit of sprucing up and a more distinct barrier from Planet Snoopy, as others have said.