What can police search for with a search warrant?

They can search for what is stated in the search warrant. In some states if they find something else, say illegal drugs sitting out in the open and easily seen, they can confiscate them and arrest the person (s) involved.

But the search must be "reasonable" relative to what they're looking for. Drugs, they can look any place you could put them. But, for example, if they have a warrant to look for a stolen baby grand piano. they can't look in any place too small to fit a piano, so if they searched a desk drawer, any drugs found would be illegally searched and inadmissible because it's unreasonable to expect a piano to be in a drawer.

But if they found the drugs in the piano, that is a legal search!

Anything that is reasonable to find with respect to the object being searched is admissible, so if they have a warrant to search for betting slips because they got tips that make them think you're a bookie, then if they find any drugs (because they are roughly the same size and can fit in the same areas) then the drugs would also be admissible.

Moral of the story: If what the officers are searching for could be concealed in small spaces, expect to have ALL your small spaces searched and, regardless of what the warrant says they are looking for, if they find something else illegal you will be charged with it.