While this isn't at all like StumbleUpon, I've been enjoying a similar sense of discovery by adding https://indieblog.page/random as a bookmark to my web browser. Whenever I have some spare time and feel like surfing the web the old-fashioned way, I click this bookmark and get a random post from an independent blog. It's definitely a refreshing and much calmer experience compared to the black-box-algorithm-driven feeds of mainstream social media.