Wobbly chairs are annoying. While a sugar packet might be a temporary solution, try using a wine cork for a more permanent fix. Wine corks, even those made of plastic, are the perfect shape for a chair leg.
Measure the cork, cut it and glue it on the end of the chair. I'd suggest not using the stained end of the cork since it may mark your floors. Otherwise, corks are soft and won't scratch the floor, so they're a great, cheap solution.
How to fix uneven patio furniture legs with a wine cork [Mother Nature Network]