- The wavy clock. Frequency proportional to a sine wave. That one sounds like fun, actually. It'd probably have to be table-driven. No sense doing actual floating point on a battery powered ATTiny.
- The lazy clock. Pick a random number of seconds and just stop. Then tick at 10 Hz until you catch up.
- The zany clock. Pick a random tenth-of-a-second within each second for the tick.
Anybody have any others?