My Loki liked to climb, although not obsessively. He used to jump up on R.'s desk, up on top of the big CRT monitor, and thence to the top of the tall bookcase, and observe the world from that altitude. I wish I could have afforded to buy him an elaborate cat-tree, but he was a HUGE cat - not fat, just big, but normally proportioned - and a climbing apparatus for him would have had to be made out of something other than heavy cardboard :-(