One method that I use is to take old cardboard boxes (for example Crate and Barrel has some dynamite ones), computer boxes would work great, grocery store boxes etc. If you go to the produce dept. of any grocery store they will gladly share the goods with you!
Once you have your boxes, be creative!!! The sky is the limit on what you can create! You can make your Cat Gym as tall or as short as you wish.....you can stack the boxes, attach them with craft glue, you can carpet them with remnants of carpet that many people have laying around (you can carpet them both inside and outside) The carpet works as an excellent way to get those front and back paws moving, grooving and scratching.
Look for cat toys that have cord (Petsmart) www.petsmart.com , (Petco) www.petco.com all have great selections of cat toys and you do not need to spend alot of money. Just secure the end of the cord (shorten to suit your needs if necessary) with crafting glue (sold at Michael's), www.michael's.com and your kitty will have his or her own personal punching bag to release those feline frenzies!!!!!!!! Also go through the countless cat toys that we all have laying around, they can be used for decoration, they can be put in hiding spots in the boxes for encouraging a great game of "Hide "N Seek".
Be sure to have some type of "comfy" area (a nice pillow, flannel blanket) in one of your "gym" boxes"....after having such a fun workout your kitty will want to take a catnap,
You can also secure a small mirror to one of the walls of the gym. (after all after working out any self-respecting cat will want to check to see how his fur is flying before venturing out to see and be seen after a sweaty workout!)
Cut holes to make tunnels, attach other items such as long, empty cardboard tubing etc to the boxes with passageways and your kitty will amused and entertained for hours!!!
The sky is the limit on what you create, there are no rules, just use your own imagination, your cat will be entertained, amused and eager to strut their newly toned kitty "sixpack"!