The GoPro Hero5 Session is about as small as an action cam can possibly be, and yet it shoots gorgeous 4K/30 footage, and is as rugged as they come. It normally sells for $299, but Amazon’s marked that down by $100 today. For reference, this was $250 on Black Friday, but included a 16GB microSD card and a headstrap mount.
If you want the touchscreen control of the full-sized Hero5, it’s $100 off as well.
Of course, the GoPro HERO6 just came out, but unless you really need to capture 4K/60 footage, it’s probably not worth the price premium compared to these deals.