You can say "Ο κόσμος χωρίς πόλεμο". This has almost the same meaning as "Ο κόσμος χωρίς πολέμους". The former refers to war as a phenomenon, as a situation, as a concept, while the latter refers to various conflicts that exist in different parts of the world simultaneously or at different times. (The two are not too different, really.)
You can also say "Μαθαίνουν χορό" if you just mean "they learn to dance"; but if you want to stress the fact that they're learning different dances/kinds of dance, especially traditional, you should say "μαθαίνουν χορούς".