Katie, I visited Castle Howard in 1996 on my first trip to Europe and enjoyed it a lot, especially the grounds which were beautiful. But it's pretty pricey now, and on my last trip in 2024, I decided not to go. As I had a choice between that and Chatsworth, I wound up going to Chatsworth which I really loved. Honestly, I don't think you're missing anything by not visiting there.
And I highly recommend Cat's recommendation for RSPB Bempton Cliffs. I spent part of the day out there on my trip and thought it was wonderful, especially standing at the top of the huge cliffs and seeing all the birds (the puffins, the cormorants and so on). It's easy to get to (it's a nice little drive there). Once you get there, there is a car park and then you walk down a slatted boardwalk to get to the cliffs. Bring binoculars if you have them. There is a small cafe there and a very nice gift shop. I spent some dollars there. ๐
BTW, I don't know if you're getting farther north, but if you do, definitely stop at Cragside House which was probably one of the most interesting places I've been to. I loved it there. The setting was gorgeous with lots of walks you can take through beautiful gardens and woods, and the interior was really fascinating.