This is just something I was tinkering with after another bottle of beer. Those pull caps looked like they had some unrevealed crafting potential, so I decided to investigate it a little bit. And this is what I've got as an result.
The title of the instructable says it all about the idea, so let's take a look at a wery short process in both of its alterations.
Step 1: Separate
First of all we'll need to separate the plastic rings from metall caps. Some robust scissors will do it just right. Those rings are what we're going to use.
Step 2: Option One
As option number one we can make a simple buckle. Just make some hole at the basis of the ring with an awl or drill it with small drill bit. Bend a pin from a piece of thick enough paper clip and secure it in the opening. And here we go, the buckle! This works for the belt-like construction.
Step 3: Option Two
For the option number two, which works with straps, just take a cople of rings and secure them in the loop at the end of strap a little bit loosely. And this is it.
To secure the other end of the strap in this type of buckle, slide it under two rings through the opening, then wrap it around the upper ring and pull then under the lower one to the outside. You can easily pull the free end of the strap to adjust the size and it will be locked then in place by the construction.
Step 4: A Bonus
And this is a little bonus that kind of fits the theme.