When not thinking about use cases I do this

https://perfectroastpotatoes.com

If simulating busyness is required

https://busysimulator.com/

Safe for work despite the name

https://bookshelfporn.com/

The physical reality of the internet is fascinating

https://www.infrapedia.com/app

https://openinframap.org/

TVTropes is a rabbit hole from which it is difficult to escape

https://tvtropes.org/

In-browser Prince of Persia. Fall horribly onto spikes like it is 1989 again...

https://princejs.com/

The O RLY cover generator

https://dev.to/rly