With Knott's, you have to watch the hours and the day you go. If the park closes early, you may have to catch the marquee attractions at the start of the day. Weekdays are usually the least quiet at Knott's and sometimes you can have the entire park practically to yourself during the first couple of hours of the day day.
Must Do's
Timber Mountain Log Ride
Ghostrider
Mystery Lodge
Calico Mine Ride
Big Foot Rapids
Extreme Scream
See all of Ghost Town if you can!
Montezuma's Revenge
Must Eat!!!
Mrs. Knott's Chicken Dinner Restaurant. Do it for lunch and you can avoid big crowds.
I haven't been to Knott's in about 6 years. (OMG! I need to go back)
That being said, I love Knott's. Especially on days when it's not too packed and not too hot.
If you're not doing the water park, you will have no problem seeing everything in a day.
Have Fun!