Nested

[Mister Six]

This is a fun way to lose 15 minutes: http://orteil.dashnet.org/nested

Basically it's a series of nested menus - like bulletin boards, back in the day - with randomised content; they start with a universe and zoom down to the atomic level (and then onwards...).

The fun bit is finding various lifeforms in the terrains and reading their thoughts, whether they're stinky cavemen, nano-equipped spacefarers or even fleas. The minds of magpies and vampire squids are especially fun.

[Mister Six]

Here's a fun thing - find a technologically advanced civilisation, go into a home dome in a living centre and find a microscreen, then read the thoughts of the nanobots inside.