touch activated light up flashing led toy.
I found this little frog at Dollar Tree, but you can also find them on ebay,
for example: ebay: touch activated color changing led light up toyebay: water activated color changing toys
Magnet activated security alarm siren, from Dollar Tree
Suction cup to hold the frog to the side of the bathtub at the desired water level
Hot knife/soldering iron
some lengths of thin wire
sugru to reseal the frog after modifications are made