The Yorkshire Sculpture Park is a full day out (and some). A small selection of the exhibits are inside, some are in the park itself, and some are in the Country Park at the side. One of my favorite bits is the deer shelter. It was once (as you can guess) a deer shelter, but they tasked James Turrell to turn it into a magnificent "sky space". Its wonderful and very surreal to sit inside the building looking up through the open square in the roof and see the clouds, or sun or rain. It gets a bit busy sometimes, but on a quiet day, its great to just sit here and clear your mind. Then walk back up the hill to the excellent cafe. You can read all about it here:
His exhibition of sculpture in "light" in 2006 was out of this world. If you ever get chance to see any of his work, you wont be disappointed.