Totally agree with rajique regarding the platforms, scratchers etc!
Ours prefer to climb more than anything, We've also got some bird feeders hanging in a bush pretty close to the run.....they will sit and stalk them out them for hours.
The run attached to the house is about 4m x 3m. It provides them with more than enough space to play in, Once you've built the main run you can just add things as you go......ideas for the interior will come with time once you see how your cat utilises the run.
Heidi found a big peice of driftwood that had been washed up on the local beach, It was like a conifer style tree that had all the bark removed. I chopped it down a bit and made a double platform out of it for our stud pen.....It looks really well and he loves it, we now call it "Uproar's snatch post" for obvious reasons
If your good at DIY you should be able to knock something together that will stand the test of time for about £200-300 depending on size and providng you already have a sound base to put it on.....you can buy ready built enclosures but they're very pricey for what they are, considering it'll only you take a few days to construct in the summer and you'll get that tailored run always wanted.....a self built run is the best option IMO.