I love the micro terrariums, they might come in handy as my room is already overflowing with plants.  I have a few planted in small fishtanks but they are larger plants coming from the top and I wedged some random shells and trinkets between the dirt and the glass, never thought to scale it down and go into the tank though.  Sweet idea.

Another thing that I have done with the little fish tanks is to make key holders that are kind of a tossback to the swinger parties of yesteryears that I hope my parents and grandparents never went to.  I collected a ton of old keys from family/friends/thriftshops, then toss them into a small fishbowl and fill it halfway with clear epoxy resin.  Let it dry and when you get home you can toss your keys/change/whatever into it, also a good way to coax car keys from drunks at parties.

Peace
Neil

The MP3 Tin is an excellent idea, strong work.  I used to use wire to turn things into buckles but they never really held up, too much dancing probably.  I finally gave up on trying to solder my own and I find some premades that are awesome.

We have a store up here called Century Textiles (or at least we had one until 5 months ago, the closest one is in Saskatoon 2 hours away now) I buy a lot of my hardward from them: snaps, rivets, etc.  And they sell thin little belt buckle backs, they are awesome.  I have converted a lot of old $2 thrift store Nintendo Controllers into belt buckles with them, as well as junk yard hood ornaments, audio cassettes, basically anything.  I use an epoxy glue and clamp them together until they harden.  It works on plastics and should stick to tin as well, if not the buckle backs have screw slots that you could always drill throught the tin and use some little bolt/nut/washer combos on.  The store that I got them from sells a lot of leatherworking tools and materials.  You might even try saddlery and tack shops. 

Peace
Neil