Rhythm
Rhythm is important so consider doing clapping games where you or your child clap a pattern, then the other one echoes it. A good example of this game is here. Pick a song and clap or stomp the rhythm to it. This is a video of Can't Stop the Feeling with suggested body movements/body percussion.
Pitch/Notes
This video is an introduction to musical notation. Talk about how every song is made up of notes. Encourage your child to make a song about something they like. They can make up their own tune, or if they are stuck encourage them to sing their song to a tune they already know.
Expression
Talk about how listening to music can make you feel emotions. Ask them about a song that makes them feel strongly. This video is a good example of how background music can demonstrate emotions.
Instruments
Talk to your child about the instruments they may have seen or heard recently. There is a game on the Sesame Street Website that has instruments that your child can play online. Have fun playing the instruments and have a discussion with your child about songs that might have these instruments in them. This video is a good real life example of what a musical group can look like.