You can buy a multitude of bass guitar gadgets, but the question you always want to ask yourself is, "Will this make me a better bass player?" One gadget that's guaranteed to make you a better player and can certainly enhance your practice session is a phrase sampler, also known as a looper.
The looper is a great tool to incorporate into your practice routine. It can take any phrase you record into it and repeat it ad infinitum, giving you a chance to listen to what you sound like without having to play at the same time. You can record a groove and check out how it sounds. Is the timing solid? Do the notes sound correct? Is the tone pure?
Eventually, as your skill level as a bass player increases, you can loop a groove with the looper and then play a melody or a solo over it as it repeats your groove, giving you a chance to really work through an idea. A looper certainly keeps things interesting in the practice room and can greatly enhance your progress as a bass player.