A quick, easy, and creative way to reuse old lens filters is to turn them into mini photo frames! Simply trim your photo to fit inside one filter, and screw another one on top to protect your picture. You can use any size or type of filter, but the clearer the better.
You can use a small filter or a rear lens cap (shown) as a base. You can even glue a magnet to the back to stick on a refrigerator or other metal surface, or use industrial strength Velcro to attach to other surfaces.
This is a great way to reuse old filters that aren't being used anymore, or may even have scratches on the glass (best for using on the rear side of the frame). And the best part is, it doesn't even have to damage the filter which can be used again in the future for it's original purpose.
Don't have any old filters that you would want to use? Find some on keh.com (P.S.- Find variety stacks of uncleaned filters under "Odds and Ends" on the site).