To introduce Rhythm, I usually do the "Sound Rap" activity with the students. I ask them to think of animal, vehicle or instruments' sounds and write those sounds in 2 verses, like that of poems. I usually show them the pattern of words like 4 sounds in a line (meow, broom, ting, click) so they complete 16 sounds in a verse.
With My grade 2 students, i often just ask them to have 2 sounds in a line to make up 8 sounds in a verse. Whoever finishes first gets a merit and a chance to go to the keyboard where they can use the head phones and recite the words they have written as rap, with the assigned register that I give them from the beat bank in the keyboard.