I only know about RULES (Easter egg) on the HP 48. Are there any others? How about on the HP 35s?
Stop watch on HP-45 :-)
The timer could be invoked by pressing RCL and then pressing CHS 7 8 all at the same time. See
here
The more recent financial/scientific fast HP 10BII+ is garanteed to hide an easter egg related to one of its creators passions.
Apparently this Easter egg is well hidden and the ones who know it are keeping the secret away from the non initiated ones.
HP 10BII+
The decimal points in the display on the HP-19c (and likely HP-10).-kby