The first night we only put 5 in the tub and it was not super interesting. Tonight we put in a whole bunch and connected them to line the whole bath.
I highly recommend this!
Then we decided to have some fun with the shutter speed...
...and then the fun continued into the bedroom...
Keep in mind these have small parts and although they are not easy to break, they can. Do not leave your kid unattended in the bath with these - or in general don't leave your kid alone in a tub of water :) If one does break, I would call poison control just in case but my friend's little brother broke one in his mouth and poison control told us that they just taste really bad and wont harm you (but don't take my word for it).
We also like to add the sesame street water dyes to the tub at night. A can of 25 was only $3.49 at RiteAid.