I like the feather idea. A lot, actually.
The problem with getting something to stick to a pen is making sure that your object (let's call it a Topper, shall we) has something long and pointy that can be affixed alonside the pen with tape. You won't be able to make a topper by merely gluing something to the "eraser end" of a pen. You know what I mean. It has to be a part of the entire pen or at least extend down the majority of it.
And you can tape pretty much anything to a pen using that green floral tape (which is basically the same stuff they use to make the flower pens). You can get that stuff at a craft store or possibly even a grocery store in the hardware section. (You know, the place where they keep motor oil and lightbulbs and tiny plastic hooks to "hold" your coffee cups).
Other than feathers or flowers, though, I can't think of anything else that comes with a stem-like appendage. Maybe some... greenery? Like a fern frond?
You could do that to all your pens and make your pen-cup look like a tiny tree.
Floral tape:
MS Paint "diagram" of proper technique:
First, get your supplies:
Next, with the feather quill/flower stem against the pen, wrap the tape around in overlapping circles until entire pen is covered with tape.
Note that my pen looks lumpy. This is not ONLY due to user error in MSPaint (even though most of it is...) it is also because your pen will probably look lumpy, and there's not much you can do about it.
I made pens for my wedding like this and none of them were as pretty as the ones in the store. :2cents: