hive of activity = a busy space
You could use it to refer to any busy place, including the space in your mind, but I’m not sure if it has been collocated with the word “mind” very often.
Here’s an interesting variation on the metaphor though:
“My mind was a hive of swarming gadflies whose stings were my remorseless thoughts, visions of her unchastity - mad, shameful, bestial imaginings.”
The Flying Dutchman
And another:
So while Reynard was avidly engaged in initiating his grown-up student in the ABCs of driving, his mind was a hive of development in progress. There were ideas taking shape, new discoveries being made (accomplishing the tricky task of opening a stubborn truck door is quite an achievement for a toddler) and a whole new world that beckoned to his childish curiosity.
thehindubusinessline.com/lif … 060100.htm
And you can qualify the original in many ways and to suit your context:
Shoreditch is a great spot for going out with Old Street a hive of late night activity.
A hive of fun activity, there is. always lots to do at the Glamis.
Hide House seeks to become a hive of artistic activity …
Rotorua a ‘hive of UFO activity’
Singapore is a hive of entrepreneurial activity.
Every side street is a hive of bustling activity.
As the Clyde - once a vibrant hive of frenetic activity, reverberating with the sound of riveting and billowing with smoke - gradually fell …
Source: Google