Depends, Those stupid decorative glass beads that people fill vases with, as howie said. Some of those are a decent size. Sand and salt I'd have the same worry about bits in ya mouth. Crushed ice would be back to a spill threat in no time. Marbles?
M n M's, smarties, skittles.
Anything that wont go off, wont give off fumes, or melt. Preferably something that isnt tiny bits that might fly into your mouth, or is loads of tiny things that will need scrubbing clean.
Hmmm
I think I'd go for marbles (and fill the rest with all my regrets).
EDIT: Worth bearing in mind that the water does at least help keep the glass cold, which may or may not make any difference to the experience. Just a thought.
