General Activity:
* Hussle: A more informal word for hurried activity.
* Stir: A general sense of movement or activity.
* Rush: Rapid movement or action.
* Scramble: Energetic and chaotic movement.
* Furry: A more lively and energetic synonym for bustle.
Specifically, referring to people:
* Throng: A large crowd of people moving about.
* Swarm: A large number of people moving together, like a swarm of bees.
* Teem: To be full of people or things moving around.
* Hum: A low, continuous sound of activity, often associated with many people moving around.
Specifically, referring to activity:
* Fervor: Great intensity of feeling or activity.
* Hubbub: A confused mixture of sounds, usually indicating activity.
* Vigor: Strength and energy.
* Frenzy: A state of uncontrolled excitement or activity.
* Clamor: A loud and confused noise, usually associated with activity.
More formal synonyms:
* Activity: A general term for any kind of action.
* Motion: A change in position or location.
* Movement: A series of actions or changes.
* Animation: Lively activity or energy.
* Vivacity: Liveliness and energy.
The best synonym to use will depend on the specific context of your writing.