Was going to make this into a how-to, but realized I forgot to take pictures along the way.
Basically... here's what I did:

Take a clean bottlecap.
Drill a hole into it & add a jump ring.
Add some glue into the bottom.
Add fun things like letters, gems, ribbon, etc.
Add some Gel Medium on top.

Let it dry for a REALLY long time. Like, Really long.
(I had set aside 2 days for it to dry before I had to mail it to the birthday boy, and the gel medium still hadn't gone clear by the time I put it in the mail - however, he received it 2 weeks later, and said that it looked great - so I guess it turned out okay) - I've never worked with Resin, but perhaps that would dry a little bit faster?

Add a metal chain.

Find some kind of other cool thing to attach (in this case, I used a foam guitar) ... Add a jump ring.

Attach it to your key ring.

Voila! All done.