@lacouvee #FluorescenceFriday for pretty microscope pictures, if you're into that.
Also:
#fensterfreitag (german-speaking, but pictures of windows or views out of them should be pretty universal)
#caturday (cat pictures, just in case you need more of those)

There also is #TardigradeTuesday which doesn't have any content, but I wish it would. Hey #Tardigrade folks, this is your time to shine!