Before I bought my Google Home Mini, I used a portable Bluetooth Jawbone Jambox to play music at home, but when I built my smart home infrastructure, I wanted to use Assistant to control my tunes while still being able to carry them around the house. Back then, Google Homes couldn't connect to external Bluetooth speakers, so I invested in a portable Sony one with Chromecast support so I could still enjoy some music while showering.
Nevertheless, a few truly portable and wireless smart speakers later came out on the market and let you interact with Assistant without worrying about cables. Among these few is the JBL Link 10, which is good but also ridiculously cheap, as refurbished units are down to $40, half the price you'd pay for a new one at most retailers.
Just like most Assistant devices, the speaker connects to WiFi and can be woken up by saying "OK Google." The Link 10 can be carried around thanks to its 4,000mAh battery, but also has a charging cable to plug it into the mains.
The units are sold directly from JBL and come with a full warranty, but you'll have to forego the original packaging for this price.