Build an indoor climbing tree for your pet! Cut several pieces of plywood, attach the plywood into a sturdy platform and cover the platform with carpeting. Secure large PVC pipes to the board and use PVC connectors to create tree branches. Wrap the pipes in sisal rope and attach a sturdy tree branch to the end of each pipe. Attach rolling casters to the bottom of the platform and set several planters on the platform to add living foliage. Cut a large bucket in half, cover the bucket halves in carpeting and attach the buckets in the tree branches for cat nap nests.
Source: IKEA Hackers