I finally got Tashi's 'upstairs' built this weekend. His pen is 8'x6' (xpen and NIC mix) and he is happy bopping around in it. I wanted to give him more variety, and a challenge, but was a little concerned about whether he would be comfortable going up and down to a second floor since he can't see. The second level is 42"x14"x14 (all NIC), built on to the outside of his pen, and fully enclosed. I hinged the top with zip ties, and closed it with pressure clips so I can open it if necessary. I covered the floor with cardboard and blankets for the moment; I may end up needing something more permanent, or at least more washable. He can reach his 'tower' by way of a ramp made of 2 NIC panels, with another one offset on the back for stability. The ramp is also covered in cardboard, and then in this stretchy brown paper stuff from Staples that is used to wrap breakables for shipping. It is very textured so he can get a good grip. It is unlike anything else in his living space, which I thought might be useful for navigation. There is a little cardboard 'lip' at the top of the ramp so it is clear where the edge is. I also placed 3 NIC panels in an "L" shape perpendicular to the ramp, for stability and so he couldn't fall off accidentally.
I was in the process of installing this last bit when he came over to help me, discovered the ramp, and ran up and down over and over. I guess I needn't have worried. He definitely likes it. 
By the way, if you buy your NIC supplies at Target be careful. I read the NIC construction advice on this site and noted the warning about the 3" squares. When I was at the shops I carefully checked in the box to make sure the squares were the smaller 1.5", and they were. However, when I got home and opened everything up I discovered that the box contained a mix of the two sizes. It took a bit of fiddling to make sure Tashi is safe.
