I love Doug's fishbowl idea! You could fill it with citrus fruits - oranges, lemons, limes...or a combination of them. If you choose just one citrus fruit, then you could intersperse a few small white flowers with them, too, for a different look; but the 'one-fruit look' looks good too...hope that makes sense. Or you could try some of what I mentioned I'd used...cranberries, red-hots, mardi gras beads, silk flowers, real flowers....you could take some real flowers, ones with large petals or real showy ones, like bright gerberas or peonies, and submerge them in water in the glass...I've done that for a centerpiece, but if you have a base on the top of it for your cake, that would look really pretty, I think. Ooohhh - coffee beans would look neat too! Or get some of that curling ribbon used for giftwrap and curl it up and stuff it in there. lol I could probably think of more, if I kept going!
If you use the glass upside down, I would suggest filling it with whatever you decide to use, then use a base to put on top of it and then flip it over so that the glass is then upside down. Much easier than trying to fill the glass once it's already upside down.