Luke loves plastic. I have to be careful when a plastic bag is just lying around because he might go in it. I try my best not to have the bigger plastic bags out lying around because I'm afraid the cat might suffocate in there (like the big JCPenney plastic bags).
Kelly and Luke both loves boxes. Basically, anything that confines them. All type of boxe, lunch box, cardboard box, shoe box. I keep some retail boxes, and the cats have found them! They go inside them and bite the flaps, so sometimes I'll see pieces of bitten cardboard scattered around the area.
We have the 24-pack bottles of Poland Spring water, and the cat likes to go in there, where the bottles would have been. I'll have to get a picture and show you guys. My descriptions are not the best.
On a slight different note, they like to lie on stuff. There could be a piece of paper on the carpet (from a small piece of notepad paper to the usual 8x11 sheet), and the cat will lie RIGHT on top of it. It's so cute that they like that stuff. It's fascinating to me.
*Sigh* The things I'd try and understand if I were a cat for a day, lol.