My 5 month old miniature poodle ate about 1/2 or 3/4 of a caramel flavored rice cake. His bowels were a little loose and he is drinking a lot of water. Should I call the vet or what should I do?

I don't think a vet call or visit will be necessary based on loose stools. It will just need to run its course. The increased water is most likely due to the sodium content. If there is no improvement after a 6 hours or he gets additional symptoms, then I would make a call.