* As restless as a caged animal.
* Like a child waiting for Christmas morning.
* As fidgety as a fly on a windowpane.
* As tense as a stretched rubber band.
* Like a ticking time bomb, ready to explode.
* As eager as a hound on the scent of a rabbit.
* As impatient as a driver stuck in traffic.
The best simile to use will depend on the specific context and the level of impatience you want to convey.