The item that's been getting constant use this year with my 5yo (just finishing his 2nd year of preschool) is the easel we have from Ikea. It has white board/dry erase on one side and chalkboard on the other, but we pretty much just pin paper up on the chalkboard side (with a couple of big binder clips) and the kids use the dry erase side. The white board side is also magnetic, and I have a set of these magnetic letter tiles that all visiting kids seem to love: http://brightideaswebcatalog.com/magnetic_teaching_tiles_...
I keep most of the same stuff you mentioned onhand. Also, shape stencils, stamp sets and ink pads. (they also have cool stamp sets at Bright Ideas, the same store I linked above - they have a retail location near my house.) Lots and lots of white paper, white cardstock, white construction paper (same store again.)
And stuff I don't buy but just squirrel away from other projects or whatever - felt pieces, fabric glue, old jewelry, buttons, jeweled glue-on things, fabric scraps, yard, colored pipe cleaners. We don't use all of that stuff all that often, but when they get on a creative tear it's nice to have a lot of things to get out that they can use. My grandma used to keep a big bag of fabric and a box of old xmas cards I could cut up and make things out of. I loved that!