Most Assistant speakers are tied to a power outlet, but the JBL Link 10 can go where you go. It debuted at a rather hefty $150, but the price has since dropped to around $80. Now, you can get a Link 10 for even less cash, as long as you're cool with a refurb. The speaker is on eBay for $49.99.
The Link 10 works plugged into the wall, but there's an internal 4,000 mAh battery that's good for about four hours of playback. You can connect it to WiFi for access to the Google Assistant. Like other Assistant speakers, you can use the "OK/Hey Google" hotword to wake it up.
These speakers are manufacturer refurbished, so you won't get the original packaging. The seller is Harman Kardon, so they come with the standard factory warranty. The $30 price drop is generous, considering that.