I've come up with two ideas for close up routines that would require gaffs. Although I don't think either is too hard for someone who does that kind of thing, they are both definitely beyond someone who doesn't, i.e. me.
The first is a gaffed shot glass which would appear full of liquid but could be handled pretty freely (for example, laid on its side) without spilling, and then you could drink the liquid completely empty. I'm pretty sure it'd have to be made of plastic.
The second is a gaffed screw bottle top -- one of those white tops from a bottle of water -- for a signed cap in bottle effect.
If anyone on this forum has any experience in making up gaffs from these kinds of plastic please email me at email@example.com.