What about a dwarf puffer?
A blue spotted puffer (aka blue spot toby)
a Pink-spotted puffer (aka Bennet's toby)
a valentini puffer
check this site out for more info:
http://www.marinecenter.com/fish/puffers/
I think the puffers have to be kept singly and will eat your CUC but they are very interesting to watch.
Or, totally on a different note: what about a mantis shrimp? Maybe a little too much to watch to eat but it's agressive and smal!!