There's something about the combination of IKEA and cats that just works. We've already had two versions of the IKEA cat litter tray and a bed for obese felines: now here's a bookshelf converted into an exercise space for kitty.
At savvycat, Liz and Jarrod detail how they built a cat run using IKEA Expedit shelving units. It's an idea you could adapt with almost any shelving system, assuming that you can resist filling that extra space with books over time. It also doesn't require any fixed elements, which is very handy if you're renting. If you can drill into the walls, check out the similar cat climbing frame.