"Framed bottle caps. I have a huge bottle cap collection, and I've been meaning to do this for some time. You can use a clear-drying adhesive directly on the bottle cap tops and press them against the glass. You don't have to put the back of the frame on and it will look like the caps are floating.
Tip: Since bottle caps are light and won't get rattled around, you should be able to use Elmers glue for this. If you want a more permanent solution, an adhesive such as E-6000 will do the trick. Just use a very small dab on the face of the cap, press into place and lay it flat until it's completely dry."