A couple weeks ago I have purchased the Xiaomi Amazfit BIP smartwatch. It is a very cheap (50$) and simple watch with a GPS sensor, heart rate monitoring, sleep tracking, step counter and an enormous battery life of multiple weeks!
The long battery life is possible through the always-on display which seems to be based on an e-ink display of ebook readers like the Amazon Kindle. In contrast to the ebook readers this display also shows a low range of colors.