I am looking for a few "easy" things teachers can implement in the classroom, e.g. playing a certain type of music (which composers are most suitable?).
https://www.duolingo.com/ is an excellent free resource that I use in the classroom to allow students to learn a second language. It allows you to set up classrooms and create assignments to be completed if you like. I simply offer it as a resource in case any of them want to learn on their own time.
Look Aa, there is nothing better than reading & listening at the same time
whether it is John Lennon's Imagine, (karokeé is nice too) any Ted Talk with English subtitles for example try
Josh Kaufman's "learn anything in twenty hours:"
https://www.youtube.com/watch?v=5MgBikgcWnY
or most TV series with English subtitles. Your best students are already watching series like Game of thrones, walking dead, Marvel or Harry Potter films in English with en-subs. TV Series have head shots which lets your students see how native speakers form sounds with their mouth and lips. If you doubt that this is important for language learning then check out the mcgurk effect
https://www.youtube.com/watch?v=G-lN8vWm3m0
Of course if I taught Spanish... Narcos would be a good place to start and there are more and more Ted Talks in languages other than English subtitled in the speakers own language.
Assigning for homework series or films is a delightful way of extending your classroom and establishing good language learning habits. Students are usually more savvy on where and how to watch shows via the internet than their teachers;-)) Imagine they watch an hour of subtitled video daily that amounts to more than 360 hours a year. Usually viewers are exposed to 300 words or so an episode therefore do the math. It is similar to spending months in a foreign country. Finally, if most of your students have smartphones using EDMODO in a classroom for pop quizzes and polls is amazing. Have fun
I think you might need to be much more specific here - do you mean: "What can teachers do to facilitate learning?" in which case there are many texts, theories and even exemplar materials to be found all over the literature. If you specifically mean: "How to teachers facilitate learning a second language?" then a fuller context would be most useful - what age? what previous proficiency to the learners have? what level of competence are the learners required to achieve at the end of the teaching?
In the UK there are national societies / associations which will help support such activities in school phase settings, for example: https://www.all-languages.org.uk/
It may be that there are similar august bodies in your own country that could offer some more focused support?
That is a very important question for teachers. All replies have a point. I would like to add that it is nice if teachers can enhance the learners' experience with the subject taught.
E.g. in language acquisition, let the learners see, watch, hear people talking the language and engage in it.