What are the other flavors of hentai?

If vanilla is a happy couple having normal sex, what is chocolate, strawberry, etc.?

Depends. You can either go with the connotation of chocolate being either scat or the dark genres of mind break, ntr or torture. Strawberry probably blood/ blood play that kind of stuff. Guro could fit both. More the swirl effect. If we go the the 'give me the chocolate hisao' then chocolate would be scat.