Mine might be a weird one but a USB speaker. My practice is 100x better with some music in the background and there’s never been a prop or mat that changed my practice as much as some good quality music without it beeping at me about charge or losing signal.
Best purchase by far.
Oh and as a taller Yogi, a larger mat that actually fits (so I’m not constantly coming off it) and some yoga pants I don’t have to stretch (someone else just started this Tall Yoga Pants (for Tall Yogis)) was huge too.