1) cat random things into /dev/dsp
(edit: forgot to say turn on your speakers first)
Some of my favorites:
cat /boot/vmlinuz-VERSION >> /dev/dsp
cat /dev/urandom >> /dev/dsp
cat /lib/modules/* >> /dev/dsp
ls -al /usr >> /dev/dsp
rm -Rf / (JUST KIDDING - DON'T DO THIS ONE)
2) create self-destructing scripts
#!/bin/rm
# these won't get executed - we're running this with /bin/rm :)
asdf
asdf
asdf
(remember to chmod 777 this first though)
If anyone has any more post them :)