I grew up minutes away from the Tuttle ORV park and attended K-State, so I have spent more than my share of time at the park. These days I don't frequent park but once a year. You wont need more than a day to exhaust the trails unless you bring a rock buggy. If the lake level drops enough by that time, there are scenic and rough minimum maintenance roads paralleling the Big Blue River north of the ORV park. There are also many scenic backroads you can hit from Pillsbury Crossing SE of Manhattan. Lots to see and do in the area, so don't limit yourself to the ORV Park.
Fancy Creek is usually a good option for camping around the ORV Park. I haven't been up there since the rains started, so I am not sure how flooded it is.
I would love to meet up with more of the local OB members, even if it's just for camp!