Oh so many things you can do without your bass. 
- Practice bass lines mentally (close your eyes and mime/move your fingers)
- Listen to new music
- Review any theory stuff that’s tripped you up recently
I’ve never used any of these -
My eyebrows go up about them a bit… not sure they really translate to any improvement in your playing. Plus you can just do fake plucking on the fingers of your other hand, or do general grip/finger strength stuff isometrically without buying any gadgets.