Definitely enough to do for one day for teens and adults. You should be able to see the park hours now or very soon, but it rarely opens before 10:00 am, so you don't have to get there super-early. If you go on a non-peak weekday, none of the lines should be long.
If the kids like thrill rides, then they will love Knott's. If anyone likes entertainment, Knott's has it, just know the weekends usually has more entertainment than the weekdays.
And on a side note, the all day dining is well worth the price and the food is actually pretty good.
You can also purchase online for date specific tickets and get admission as cheap as something like $35 for adults.
Also, on the dining plan they have also added a new premium all day dining plan that includes a souveniers bottle you can refill all day for free and you can get a meal (entree and side) or snack every 90 minutes. The premium plan is cheaper than buying both the refillable cup and all day dining and has more options available.
We haven't been in years, but we're thinking about going this year for a day. I don't do roller coasters, but I remember enjoying Knott's when I was a teen. The log flume was my favorite! I'm trying to figure out if there's enough there for someone who loves Pirates and Big Thunder but hates Space Mountain, the Matterhorn and the Incredicoaster.